There could be multiple different causes.
Jane falling could've been an accident, caused by her tripping, losing her balance, being rushed down the stairs by a crowd, etc.
Her falling could also have been caused by somebody with malicious intent -- that is, somebody might have pushed her down the stairs.
What is the context of this question? How did Jane fall down the stairs? Not the cause, but the imagery. Maybe I could provide a better answer?
I hope this helped anyway. :)
Have a great day/night, stay hydrated, get the proper amount of sleep for you, and eat healthy foods.
