Do you have a relatively new model, or is it a number of years old? Some years ago (five? dunno ...), BV found that a particular bear had learned to pop the tops off of their cans and so changed to a deeper lid, one with more threads. With the top sitting flat on the ground, the old one was maybe an inch or less. The new one is (off the top of my head, I'm lazy this morning) perhaps a couple of inches thick.

I'm surprised to in either case, but even more surprised if it was the thicker (okay "taller") lid and it was screwed all the way down tight.
And I assume when you say that the can was "upright" that means the lid was up.

I certainly have had water come into the black Garcia canister if left upright; with a Garcia, a person living in a rainy climate quickly learns to set it lid-down and/or wrap in plastic. For a BV, however --- not easy to get water past all of those plastic screw threads. Your explanation (significant change in air pressure from when you closed it) is as credible an idea as I can think of. You would think that you would know if there was a crack or a hole in the plastic!!
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Brian Lewis
http://postholer.com/brianle