The cracks you describe come from the house settling. Depending on the type of house, construction method, soil compostion, and a lot of other things, different houses settle differently.
My guess is that this will continue to occur until the house comes to rest. This would only get to be a serious problem if the door starts to be too tight to open or close, then you might have more of an issue other than cracking.
You didn't mention if this was an interior or exterior door. Interior doors, don't typically hold weight.