In Northern climates, rain water will accumulate in the steerer tube by seeping around the quill-stem and will then expand when it freezes. This will sometimes cause a fracture of the front fork.
In the early spring a couple of years ago, the neighbor kid had the front fork snap-off at the base of the steerer. Right in front of the library he crashed and several people witnessed it, thankfully he was not going too fast at the time. He had a little endo and he bit his tongue a little but otherwise not hurt bad enough to go to the hospital. His parents asked me to look at the failure and it was obvious that it had expanded and cracked. The steerer tube was split and expanded vertically as well. Their bikes lay in the yard all year, so this is how I explained it. (This failure was actually on an old USA made Schwinn, and I have seen this a few times over the years)