Not normal. Every great once in a while, I manage to confuse mine and set it off. For me, the sequence seems to be:
1. Arm alarm and wait a while.
2. Open the trunk (with the remote) - get something out or put something in.
3. Attempt to close the trunk, but don't get it completely latched closed.
4. Alarm goes off as trunk lid opens back up.
I think it closes enough for the system to think it is locked back down again, then the alarm is triggered when the trunk open switch triggers as it reopens. In your case, I wonder if your trunk switch is getting flaky?