If you have constricted drain or sewer pipes, water can slowly empty out of the pipes when not in use, such as overnight or while you're at work. Then as more and more water goes into the drain from various sources, sinks, showers, whatever, the sewer line and the pipes fill up faster than it can exit past the blockage. The water could be filling the lines to the point where it reaches the hole in your shower drain, then leaks out.
Imagine a horizontal pipe with a hole in the top, for example, where water only reaches the hole when the pipe is full but not when a stream of water is flowing along the bottom of the pipe.
That could explain why it doesn't happen all the time. You can test this by running a sink continuously for a while and seeing if the water starts to back up and the drain empties slowly. Then go turn on the shower and see if your leak happens right away. Conversely, if the house has been unoccupied for several hours and nobody has used any water, and then you run the shower and there is no leak, that suggests the pipes were empty and haven't filled up yet.
If that's the case, you might mitigate the problem by having your sewer line cleared. Your leak will still be there, but if the line is not blocked it shouldn't fill up after a lot of usage, and it might then not leak.
But that only puts off the day when you will have no choice but to open up the walls and fix it. Clearing your sewer line is good preventative maintenance anyway, so it wouldn't be a total waste if it doesn't help with the shower leak.