I'm not sure but switching to a macro should fix your problem. The other reason to switch is that sometimes 1/4 or 2/5 toons are in combat so when you mount up... only 3 toons mount up. If you wait a second and hit a normal mount button again, then the mounted half of your team will dismount while the stragglers mount.

That's why I always use mount (with the ever important stopmacro[mounted])/dismount macros rather than just keybinding a mount.

If I had to guess why its warlocks causing this problem, I know that in some instances lock/pally native mounts are different from other mounts (for example party cast bars will show a felsteed mount and not a normal one). Whatever you are running into is probably some behinds the scenes glitch caused by this (even though I know you are using a flying mount not a class specific one).