بسم الله الرحمن الرحيم
و الصلاة و السلام على أشرف المرسلين
و على اله و اصحابه أجمعين
The Hadra is a specific characteristic of Sufi. It is the symbol of the Gathering of the Most Beautiful Names (asma’Allah ul-Husna) inside the letter “Ha” of the Name of Majesty “Allah”. For those who got their inner vision opened by the Kaf of “Kanaka tarah / as you were seeing Him”, It is the symbol of the eternal handful (qabda abadiya) manifested through the pre-eternal divine Beauty. For them, Malakoute became Mulk, and what was occulted became visible through awakening state vision. Due to this, they tasted the exquisite fragrance of Proximity (Qurb) and they felt the breeze of Fulfillment.
Mount Uhud shook out of Love under the steps of the Beloved ﷺ. It shook in response to the intensity of the Muhammadan Beauty, and that until the Messenger of Allah ﷺ has calmed it down.
Abu Hadid as-sa’idi reports, “We were coming back from the holy battle of Tabuk in the company of the Prophet ﷺ and when we arrived near Madinah, He said, “This is Taba and this is Uhud, this Mountain loves us and we love it” . It is obvious that main reason of Mount Uhud tremor was only Love. Consequently, quivering and swaying are visible manifestation of Faith (Iman). In this sense a poet has said:
Do not hold it against Uhud because it shook
When he walked on it, this movement was a visible sign of Faith
And it was nothing more than a remedy
Uhud should not be reprimanded because it is a Lover
And Passion is never as intense as during the time of the beloved’s appointment
In reality, this movement was the expression of the Hurricane coming from the warmth of the desire to encounter Allah. This Hurricane is accompanied with hopping, tears, shivers and even with faint. Have you ever seen stones that fall down from heights because of fear of Allah?
Allah ﷻ says, “Then your hearts became hardened after that, being like stones or even harder. For indeed, there are stones from which rivers burst forth, and there are some of them that split open and water comes out, and there are some of them that fall down for fear of Allah. And Allah is not unaware of what you do.” 
As the different tones of voice are closely linked to the degrees of the Spirit, tremors and swaying are natural reaction to beautiful sounds, even for animals and minerals which have no Spirit. In reality, only the bodies that have no heart to fell the influx of deep meanings have no Spirit. Allah gave us an example in the Quran: “If We had sent down this Qur’an upon a mountain, you would have seen it humbled and coming apart from fear of Allah. And these examples We present to the people that perhaps they will give thought.” 
If the Quran was sent down upon a mountain it would have humbled and coming apart despite its strength, its rigidity, so what would be the heart state of sufi in such circumstances? In reality, it is not possible de contain the intense feeling of Proximity (Qurb). Thereon, Ibn Taymiyya wrote in his “Fatawi”:
“When Imam Ahmad have been questioned on this subject, he answered, “The Quran was being read in Yahia ibn Sa’id al-Qattan’s Presence when he fainted. And yet, if someone was able to push back that state, it would be Yahia ibn Sa’id, I have never met someone more reasonable than he.”
“We could also report that something similar happened to Imam Shafi’i, as forAli ibn al-Fudayl ibn Iyyad’s story which is well known. Lots of people we cannot doubt on.“ 
His pupil, ibn Qayyim also mentions the issue of “Hal” :
There is a disagreement on the issue of Swaying (Tawajud), is it lawful or not? Scholar are divided into two groups. The first said that it was unlawful from a legal point of view because it was s considered as showing off with few natural. As for the other group, they said it was lawful from a legal point of view but only for those who are sincere and verily touched by the spiritual allusions. The Prophet ﷻ said, “Weep, and if you cannot weep, then pretend to weep”.
So it means that, on the one hand, he who sways to show off or to access something, or because he follows passions and Self, it is legally unlawful for him. On the other hand, if someone sways in order to cause or to trigger a spiritual state (Hal), then it is legally lawful and it depends on the person and what we know about his sincerity and Purity (Ikhlass) 
He also said “If swaying (tawajud) was allowed or recommended for all people of Dunia who are only guided by their passions, then no one would have ever tasted the savor and the joy of the gathering of the hearts with Allah…no one would have ever tasted the intense desire of His Encounter. Only the one who tasted it can understand, because only the one who saw shining in his inner what is shining in your inner can understand. May Allah grant he who said,
“O my friend, seest thou not their fire?
He answered: You see what I do not
The flame (of Love) quenched your thirst but not mine
And you were able to see in your heart what I cannot see.” 
 Hadith 4097, Sahih Bukhari
 Surah al-Baqarah, verse 74
 Surah al-Hashr, verse 21
 Majmu’at al-Fatawi, ibn Taymiyya
 Hal : spiritual state
 Madarij as-Salikine, Ibn Qayyim al Jawziyya
 Madarij as-Salikine, Ibn Qayyim al Jawziyya