What is a phase difference between voltage and current?
For reactive loads (capacitive or inductive) the current sine does not coincide with the voltage sine. For inductive loads the voltage leads the current, for capacitive loads it's the other way around. 



That means that the zero-crossing of the voltage for the inductor comes some time before the zero-crossing of the current, and that the current is not zero when the voltage is too.
And that's the reason why zero-crossing switching is not recommended for reactive loads.



image from this excellent site.