As stated in the pop open medicine link in the FAQs, any oil or solvent on the latch/shelf can cause pop opens, I've often wondered if rain would cause it, but you're the first to report a problem, you might try it at the range to see if it will under controlled conditions, it may just have not been closed and locked up properly tho too, trying to close the action quietly while hunting isn't conducive to closing it properly to insure proper lockup.
Tim