I think what happened is the following. The container has a top with rubber that contacts the sides. You put it into the container and in contact with the water, so there should not be any mold/mould. There’s a center hole, so as you push the top down to get to the water, the air can escape.
For my first set of pickles, I added some glass weights I have to the top. That worked well.
When I was fermenting giardiniera, this must’ve produced more gas. I had two weights on the top, but it got pushed up on one side. Then, I rearranged the weights, and it got pushed up on another side. Finally, I put 4 weights on it, and it still got to be askew.
Then, when I went back to the pickles, I figured the weights might not help, so I did not use them. And there must have been a small amount of gas, just enough to slightly dislodge the top.
I’ll go back to using the weights, although for something like giardiniera, I might have to “burp” it daily for a while.