She tried to go limp as she fell, since she was really quite used to falling and tripping and things like that. She did it all the time, and was pretty good at landing so that she didn't hurt herself all that badly. Long ago, people had stopped trying to protect her from her own clumsiness, since it was somewhat of an exercise in futility and wasn't really worth putting much energy into.
This was the second time he'd tried to help her, and despite the fact that she knew she was going to hurt him very soon by landing on him, she couldn't help being touched by that fact. Of course, she didn't have much time to think about it in depth before she was crashing down from her sadle, her leg twisted uncomfortably as she landed right on the slender man's chest.
"Ow! Yeah, I... hold on a second, I think I can get my foot out if you just... yeah, like that."
She beamed at him as their mutual maneuverings managed to slide her foot just a little. Her leg fell free, and she grinned down brightly into his face, just as if she hadn't just just crashed down onto him from several feet up. She really was oblivious about some things.
"Hey, Marcus! Nice to see you again! Horseback riding is really brilliant, you should try it sometime."
The horse looked down at them and snorted, apparently stating his opinion of the whole thing. Tonks found this very amusing and started laughing, forgetting for the moment her position perched on top of him.