Cracks in bathroom sinks can be caused by several different things such as dropping something hard into the sink, stress from hot water such as thermal shock etc.
If it's a fiberglass sink it could just be the hot water going from hot to cold on the cold sink.
Or like mentioned above something hard could've dropped into the sink can cracked it.
If it's a glass sink or porcelain sink then thermal shock or something hitting it could've made it crack.
Or vibrations in the house such as an earthquake that caused the sink to shake could be another cause.
If it's cracked a little and it's fiberglass it's easy to repair the crack with a fiberglass resin buy porcelain that is cracked is harder to fix.