I would not be overly alarmed by it.
It has happened to a few people already - particularly when the electrical load is unusual. During the summer it will happen when the AC (auto is on) and you're in alot of Stop&Go traffic.
During the colder winter months it will also happen when we have the defroster on with an agressively high warmup temp set. Additonal accessories added to the vehicle will also worsen the battery depletion rate thus increasing the chances of having a nasty forced regen such as the one your describe.
Yes, the IMA system will do what it must to protect the battery SoC. However you can also help reduce such events by taking a more active role in the management of the battery'e SoC - In the end, it is all traced to what we do.
Cheers;
MSantos