This is a bug for players using touch screen.

I don't really know how to explain this bug, but basically if you tap very fast in the area that you use to place/break/rotate and have your other finger tap in the area that controls where you move (for me in the bottom left), you sometimes break or place blocks in that area (bottom left).
This is better shown in a video so I recorded a clip:

https://youtu.be/BnTBs09hBxE

Some rant:
This bug is annoying in both survival and creative especially when building. When you are placing blocks, you can accidentally place one in the bottom left. When you are breaking, you can accidentally break the block on the bottom left.