D. Rex
Magic Eight Ball
Unless Wlfsamurai makes a special exception, in general you can't reflexively dismount when someone attempts a gambit against you. There is an 'unhorse' gambit that deals one flat level of bashing and leaves you prone. If you could reflexively dismount to negate it, that would probably be spelled out. You could voluntarily suffer an unhorse gambit to avoid being grappled?
I could argue that while standing on its head and not in the saddle, and it having been forcibly yanked out from under her. That that might qualify for a voluntary unhorsing?