possible water splashing around in the float bowl of the carb. I used to do small engine repair, its a different animal for sure but the comon factor is that a mower will sit and run all day with water in the gas. The second it sloshes by movement the water droplets get sucked in and it dies.
Water will puddle to the bottom of gas like clear marbles in a the bottom of a glass jar. Sitting still, your fine. Move and your stalling w/wet spark plugs.