What is the reason of having an unstable terminal voltage on the loosely connected 2 or 3-pole circuit breaker terminals?

Loose contacts result in air gap. Air contains CO2 which converts to carbon during arcing and this carbon deposit on the current path increases the resistance. This causes voltage drop due to IR losses.

Compounding to this there is local heating of conductors close to the loose contacts and this further increase the resistance as resistance increases with increase in temperature.

Typically such problems have a cascading effect which over time may lead to frequent tripping of the Circuit Breaker and permanent fault and can even result in damage or derating of the Circuit Breaker which may have to be replaced.

If the Terminals are Loosely connected the potential difference between the two terminals are varies, Hence there is a voltage drop ,the current will increases and the loosed terminal get burnt...

The loose connections may cause sparks  upon load on that wire,  If the wire is half connected then it will create air gap,  as Ramindera Sunder said above, and as we all know that wire consist copper and in open air, if there is copper then it will turn into copper sulphate, we usually call it salt, and due to loose connection, the wire will get hot upon load, so the heat will make the carbon and that carbon will cause resistance. An electrician can cut the wire and make a new connection, or simply rub the end with sand paper.

spend some time to tighten the connection you will find every thing is ok

LOOSE CONNECTION WILL LEADS TO PASS OVER CURRENT AND VOLTAGE DROP. SO THE TERMINAL VOLTAGE IS UNSTABLE.

In Question, there is already an answer present.

Loosely connected means circuit Impedence Balance shall be disturbed & hence the Voltages.