What causes extra toner? Does it happen often? What makes it transfer, and is there a point at which it’s no longer transferrable?
Could be a problem with the printer itself not heating the toner sufficiently when the paper rolls across the drum. Could be poor quality paper that doesn't accept the toner sufficiently. Could be any number of issues.
Does it happen often?
It shouldn't but it happens often enough. It depends on how much the printer is used, the quality of the paper, the quality of the toner itself.
What makes it transfer,
Heat, humidity, and pressure.
and is there a point at which it’s no longer transferrable?
It's less likely to happen after the toner dries (i.e. cools) but if the paper or toner is poor quality then it can still transfer even when it's dry.