There are huge numbers of reasons this could have happened, and you may never know for sure. My advice is to keep a comprehensive diary of blood sugars, insulin, food and exercise so you can see if a pattern emerges.
An unexpected hypo could be caused by:
Changing your injection site. If you inject in the same place all the time, insulin can be absorbed poorly/unpredictably. Changing the site can lead to a reduced requirement.
Accidental double dose. I have had a hypo a few times before, and realised it was because I administered my insulin twice.
Inccrrect nutrition information. If you counted the carbohydrate content in the meal, you can still run into problems. What most people don't know, is that food companies are allowed a 20% margin for error on their food labelling. e.g. If you thought your lasagne contained 100g of carbohydrate, it might actually have had 80g, or 120g.
Long acting insulin playing up. I'm currently seeing a fair few hypos, and I think my Lantus is to blame. Sometimes, it can decided to act more like short acting insulin for some people.
Insulin still in effect from a previous meal. When was the last time you took short acting insulin? It can stay in the system for about 5 hours, so a previous dose might still have been lowering your blood sugars.
There are probably a number of reasons that I forgot to mention, but those are a few things to consider.