One moment she was trying to figure out if she could leap to the next tree to escape her attackers, hopefully before they brought the tree down with her in it, and the next...there was a huge explosion, shaking the trees and leaving her grasping for purchase amongst the branches and leaves. What in the... She turned to look behind her, hands going to the trunk of the tree and her hair starting to come out of its carefully crafted braid to fall about her face, and her wide eyes took in the sight of battle - a myriad of people in various states of armor and weaponry, falling into formation and attacking the darkspawn, which were answering with gusto and dying easily. It was a lot like the rebellion, actually - only those who were winning were the Tevinter's soldiers and the darkspawn would be the elves dying by the handful.
Watching the battle further, one hand clutching a branch while the other had a strong grip on her bow, she saw one man suddenly run off into the darkspawn - he yelled at someone else, "get the girl!" and suddenly she was wondering what he meant by that exactly. And then he went running off, banging his sword against his shield and screaming something about being delicious... Crazy, suicidal shem was the only thing going through her mind at the moment, but he was drawing away the darkspawn enough that another shem, this one on a horse, moved up to the tree she was trapped in and offered her a hand down, as though she couldn't get down on her own. She looked down at her, looking her in the eye as the shem stared back up at her and stood up in her stirrups. Arya's mouth opened as though to reply, but she could think of nothing to say.
There really wasn't a lot of time to think about it. She could stay in the tree and wait for the battle to wane, but they were already expending forces to save her; forces that could be used to save their own skins, and she couldn't not admit that she wasn't grateful for the help. And despite her misgivings about humans, right now they fell on the wayside of the more imminent threat of the darkspawn; and like many other times in her life when she'd been faced with a life or death situation, she chose and hoped for the best.
She jumped from the tree, grabbing the shem's hand and landing behind her on the horse as softly as she was able; she'd never ridden on a horse before, but she'd seen them numerous times. After all, it wasn't like she was the one with the reins, the shem was taking care of that, and Arya grabbed on for dear life with her free hand, her bow still in the other. For a moment she wondered if she might be able to shoot from the saddle, but the creature took a few steps forward and she felt a lurch that reminded her of a ship. Any notion of fighting from the saddle fled from her mind and she concentrated on staying on the beast instead.