So, with anything that isn't a primitive type (e.g. int, bool, etc), there's a chance that assignment is going to require memory allocation or something similar. If that's the case then there's a chance of bad things happening (e.g. a out of memory error and the program being killed).

More commonly, if you look at things like c++'s unique_ptr, assignment will do a lot of things in the background in order to keep the unique_ptr properties consistent. Rust and other languages probably do similar things with certain types due to semantic guarantees.