There can be any
number of reasons.
It isn't uncommon
for states to add, change, or delete the conditions that must be met for eFiled
returns to be processed between tax seasons.
So, the state's eFile requirements may have changed since you last filed.
On the flip side,
you may have an issue on your tax return that hasn't previously triggered a
state reject code. Something different
you've entered, for example, or perhaps a certain figure met a threshold that
hadn't been breached before.