Keys can break off inside locks for several reasons usually related to the key material or the lock condition.
Old or Heavy Used
Over time keys that are used frequently or on multiple locks are subjected to more and more problems which can eventually lead to breakage, also old and rusty locks can make it harder to turn the key which increases the risk of it to break.
Quality of Key
The quality of the material used to make the key plays a significant role as cheaper or softer metals may bend or break more easily.
Forced or Incorrect Insertion
If a key is inserted incorrectly or forced into the lock it can put stress on the key and turning it too forcefully when it’s not fully inserted or when it’s stuck can cause it to snap.





Weather Conditions
Extreme temperatures can affect both the key and the lock as cold weather in particular can make metal more breakable increasing the chances the key would snap inside a lock.
Lock Condition
A lock that is not well maintained can contribute to key breakage as rust, dirt or grime inside a lock can block the key path and increase resistance leading to potential breakage.
Design
Some keys or locks may have design flaws that make it more possible to break for example a very thin key or a lock with a particularly tight keyway can increase the risk of breakage.