One thing that is obvious you do not understand is how much expansion can be absorbed within the plumbing system of the building. Depending on the type of piping, it could expand considerable which would mean it took that much longer for the pressure to build up enough to cause the prv to release. Once it released, that cycle of the entire plumbing system being pressurized all over and realize, for the prv to pop, the entire plumbing system in the house is being raised to that pressure.

it is not an instant reaction.

Add to that the use of hot water would mean the water in the heater would start back at the base temperature and would need to be heated so much it caused enough pressure to trip the prv.

Unless you have some proof they caused this problem, pondering whether they took a hammer to the thing is simply ridiculous. Water heated thermostats fail. It's just that simple. Your plumber told you yours failed. Ask him if he saw any signs of tampering.