can you trip a thermal overload relay? if so how often does it happen and how can it occur?

Yes, the ability to be tripped is the whole purpose of a TOLR.

When motors don't reach full designed speed the motor will draw excessive current. Excessive current creates excessive heat in the conducting components that feed the motor. OL's are installed in series with the load, the heat (in old style) thermal overloads tripps the relay. Some of the newer TOL are solid state, I don't know how much actual heat is detected.

An overloaded motor, inadequate voltage (often caused by inadequate wire size), a bearing failure in the motor or bearing in a motor driven device like a compressor are all types of failures that can slow a motor down below designed speed and trip an overload.

Incorrect selection or setting is also often a cause for tripping, particularly if the wrong service factor is applied.

It depends on what it is protecting. If it is on motor. Stalling or overloading the motor will trip it.

In most cases tripping it on purpose won't be easy,
so unless you Really need to test one...

The idea is quite simply that when the protected equipment
gets too hot for considerations of safety, it is shut off.

It should Never happen under normal conditions.

A thermal overload relay typically consists of a bimetallic strip that restrains a spring, keeping a set of contacts closed. Overload current gradually heats the bimetallic strip, allowing the spring to open the contacts, disconnecting the circuit. The higher the overload current, the quicker the circuit disconnects. Some overload relays reset automatically, others require a manual reset. If current above the rated overload current of the device will cause it to eventually trip. If the device has to be manually reset, then the trip happens only once until the relay is reset. The relay has to cool down before it can be reset and this takes time. If the relay resets automatically, then the time period and frequency of trip and reset depends on the magnitude of the overcurrent and length of cool down cycle.