Perhaps the button on the key fob is broken inside, but occasionally sends a signal. Alternatively, is there a button for trunk release inside that is getting stuck or needs to be cleaned?
How often does it pop open? If regularly, I'd disconnect the key fob and keep it out of range of the vehicle for a while, to rule out if it's the key fob (and check the trunk release inside if equipped).
If not the key fob, perhaps the lock/latch mechanism on the trunk needs to be cleaned/greased. Hopefully it's not worn out.