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.