1. A thermal switch can be faulty, if so you may get the problem again e.g. goes off at a lower pressure than it should

2. A thermal switch can sometimes be sprayed with steam/water from a leaky vacuum breaker or safety valve (often might only happen for a minute or so when switched on and is up to temp...this causes them to track, fail and/or short

3. You have had a high temperature event (caused by sticky pressurestat, or some fault)...often though the safety valve lets by first then the thermal breaker. your safety valve (thing with blue top) is a 2 bar one, so it vents at around 134C, the thermal breakers are usually set to trip around 140C or 145C. However, it doesn't look like your safety valve vented, it all looks clean around there. Also if you had had a high temperature event and both limit stats were fault free...both would probably have tripped! The tripping is purely a mechanical function.

4. Sometimes a loose connection on one (thermal breaker) can heat up the inside to the point where they trip, check for loose connectors and keep an eye on it for any connector that starts to go brown

What they don't tend to do is spuriously trip, there is always an underlying problem, that usually gets worse.. keep an eye on it, see if the pressure remains within a normal range when it's on. If the same limit stat fails again and again, you could either replace it...or simply bridge the circuit and run on the remaining good limit stat. not many machines have 2 on the same boiler, it's really belt and braces stuff and probably necessary TBH.

P.S. If I was designing a machine with 2 limit stats, I certainly wouldn't put them beside each other one would be on the top....the other on the bottom, or near the heating element base.