2 most common causes...
• Either the engine is over-heating due to clippings captured in the cooling fins, Or...
• Fuel tank breather (usually inside the fuel cap) is blocked and won't allow fuel to flow to carb due to vacuum inside the fuel tank.
But others are reporting a mercury safety switch is mis-aligned. This switch shuts off the engine should the mower over-turn. This is a new requirement and the switches and/or installation may not be fully understood, or may simply be too sensitive.