1994 Chevy Suburban Lose of transmission fluid

I have a 1994 Chevy Suburban. While driving one day I went to pass and noticed oil out back of truck. We looked under truck and found trasmission fluid pouring out. I found a plug behind the dip stick had partlly blown out. Pushed it back in and spread snap ring. Replaced fluid and drove couple days and same thing happend. My question is what is causing this and what is behind this plug. No problem with shifting or anything.