Sounds like your bass player decided to go to the bar and get a drink mid-song.

I would definitely fire him/her except it sounds like he/she is at least consistent.
I've actually been in a band where that happened. The guy was a guitar/fiddle player and was playing the fiddle. He simply put it down and went to the bar and bought and drank a beer.
His explanation was, he was thirsty. If it had been because he really needed to visit the bathroom.... ok, we could have understood that but, nope..... he "needed" a beer. He was fired.
Once you've played in a few bands, you've seen and experienced practically everything.