Why the Wall fell

Potential Spoilers Below

It fell because it is a tieback to The Wheel of Time and the Domination Band.  Rand the Dragon Reborn was collared with it and was under the control of Semirhage.  She thought she had everything under control when he was able to free himself using the True Power and the following ensued:

The Domination Band

Rand al'Thor (The Dragon Reborn)

Semirhage

Semirhage looked utterly shocked. “But . . . that’s impossible . . .” she said. “I felt nothing. You can’t—” She looked up, staring at him with wide eyes. “The True Power. Why have you betrayed me, Great Lord? Why?”

Representation of the Great Lord

Rand raised a hand and, filled with the power he did not understand, wove a single weave. A bar of pure white light, a cleansing fire, burst from his hand and struck Semirhage in the chest. She flashed and vanished, leaving a faint afterimage to Rand’s vision. Her bracelet dropped to the floor.

An Aes Sedai using Balefire

Elza ran toward the door. She vanished before another bar of light, her entire figure becoming light for a moment. Her bracelet dropped to the floor, as well, the women who had held them burned completely from the Pattern.

In The Game of Thrones instead of the Domination Band they have something called Dragonbinder.  It is a horn that Euron retrieves from Valyria.  With the horn he tells the ironmen that he can bind dragons to his will.  They don’t cover everything in the TV show but is this a hint that the Night’s King will get his hands on this horn and bind the Dragon(s) to his will?  If you are a book reader you may remember what one of the glyphs on horn reads: “Here it says, ‘No mortal man shall sound me and live.’ ”  The Night King from the TV show is definitely no longer a mortal man.  The rest of the conversation goes on like this:

Dragonbinder

Euron

The Night's King

Bitterly Victarion brooded on the treachery of brothers. Euron’s gifts are always poisoned. “The Crow’s Eye swore this horn would bind dragons to my will. But how will that serve me if the price is death?”

Victarion

“Your brother did not sound the horn himself. Nor must you.” Moqorro pointed to the band of steel. “Here. ‘Blood for fire, fire for blood.’ Who blows the hellhorn matters not. The dragons will come to the horn’s master. You must claim the horn. With blood.”

Moqorro

Was the trap set by the Night King to get a dragon the answer to the riddle “Blood for fire, fire for blood?”  By taking the dragons blood and blowing Dragonbinder did he bind the dragon to himself?

Now to get back to my point was not the Ice Dragon’s fire reminiscent of Balefire (i.e. a bar of pure white light, a cleansing fire)?    You can read my entire theory about how Dragonbinder is an homage to the Domination Band here: My Theory

A bar of pure white light; a cleansing fire?

I suggest that the Ice Dragon's fire is an homage to Balefire; a bar of pure white light; a cleansing fire.

Comments encouraged.  Love to hear the ideas of others.  Most believe that since I present my idea’s as “fact like” I’m not open to change my viewpoints which is far from the truth.  I simply look at the information presented and go from there.  If you can shine a light on another way of thinking that opens the door to debate.