It is actually not a glitch. It happens to all of us. I know, it's really annoying but how else would we be able to dance around if not with our feet on the ground? Not that I would mind seeing Erin giving a little "5 seconds party" on her Kirin.
While the people are celebrating the "birthday" of Wizards 101 with the yellow present boxes that show up in random areas, I notice that when I "open" the present it takes me off my mount. I can pick up reagents and open silver chests while I still stay on my mount, but not when I open the yellow present boxes. Is this intentional or a bug with the Birthday Celebrations?
Okay I still think of it as a glitch. When I get on a mount, I want to stay on the mount until I decide to unequip it. Now, it would be one thing if we did the dance and then got back on the mount when starting to move again (temporary dismount) but that's not the case. Also would be one thing if there was a keyboard shortcut to remount the last mount being used (if it's still in our book). Again, not the case.
There is also the lack of consistency. If someone is riding someone else's mount and is the one to open one of the gift boxes, then they remain on that mount. What it comes down to is, you are punished if you are on your own mount and open one of the presents because then you have to open your book, choose the mount again (if you have more than one and it's not the first on the list) and then mount it again.