If your load is close to the eating of the breaker, such as pulling 14 amps through a 15 amp breaker, it could get a bit warm
However, the much more common reason this happens is because people use the breaker as a switch, which it isn't intended to do. Repeated cycling of the breaker causes it to wear out pretty quick, which will increase internal resistance, which causes heat and premature tripping.
1990 Ford F350 CCLB DRW 7.3 4x4
1990 Lance LC980 truck camper