From now on I'll bet that the last thing you do before opening that cap is ensure the valves are closed
Almost ran into #1 myself... During the PDI the guy doing the walkthrough opened and closed all of the valves to prove to me that they worked, so I knew there was at minimum water in the pipe. Needless to say, I stood as clear as I could while taking the cap off, but still trying to catch what I could in the sewer pipe vs. letting it run all over the ground.