Subject: Why did the lid on the milk jug pop up and down?

This is on the top 10 list of strangest things I've ever seen, and I'm dying to 
know what caused it. I listed the area of science as "Physics" because if I knew 
that, I'd have an idea of what's happening. 

This morning I was making biscuits, and since I never measure anything, I left 
the milk out next to me in case I needed more. It was skim milk in a regular 
clear plastic gallon container with the type of lid where you pull off the 
little plastic strip around the bottom and then it just pops on and off. 

Instead of popping it on while I was mixing, I just sat the lid on top of the 
opening without pressing it on. I started hearing a little "plip" sound. So I 
started watching the milk jug. The lid would rise until it was almost off the 
jug on one side and then fall back down. It was so weird. So I took it off, 
poured more milk in, did the same thing, and it happened again! I'm so curious 
as to what caused it! 

Re: Why did the lid on the milk jug pop up and down?