There are apparently multiple sources for the gas bubbles. Some are even contributed by the fish themselves. Check out this article:
Fish bubbles – what to look for and what they really mean | Advnture
As the article says, agitation of the water column can cause greater release of the lake bottom bubbles. I imagine that an outboard motor or even a trolling motor on spot lock can cause enough vibrations to agitate the lake bottoms. It would be interesting to see if the bubbles are reduced if one ties the boat off to a submerged tree or anchors with a raptor of conventional anchor instead of continuously running the trolling motor at a fishing spot.