That was a problem with those early antenna boots, but there's nothing underneath to hold it in place - it's meant to be just a push in and twist to lock action. However, once the boot ever comes off, it seems they really don't want to work so well again!! And now that your rubber boot has become old enough to deform a little,
and it's already come out once, it's almost certainly gonna keep on doing it unless you glue it in or replace it with a new one - and good luck getting a new boot by itself!
Still, as I mentioned, that 'old boot coming off' problem
IS easily solved without replacing it simply by 'gluing' it in - just put a small dab or swipe of clear silicone sealant into the groove in the base of the boot where it is meant to lock into the panel; push the boot back into place and twist it to engage the little 'locking slides' built into the base of rubber boot; then put a piece of duct tape or 100mph tape on it to hold it in place for the next couple of hours while it dries. It doesn't need to very be much silicone, a
tiny pea sized dot has worked on mine, and after being a pain for most of 2014, it's stayed in place without further issues since early 2015 when I did the silicone thing; plus, I haven't had anyone come back to tell me that any of the numerous others I've fixed the same way have ever come off again... But on those few where the antenna has needed to come off again for whatever reason, all it takes is a simple twist to break the silicone bond and it's off!
Go on, Do It! You know you want to!
