Only suggested the time because as I said reading the posts we seem to get more and more impatient and plant things earlier and earlier. Eventually we end up planting too early and the plants do what they want not what we want. An onion set is a small onion so it has already grown from a seed to what we/you plant so there is already a growth time factor built in that we seem to forget.
If seed to set size = 4 months growth, then Sept to Feb = 5 months growth then by Feb the onion has had/seen 9 months of "growth". Is that enough for it to think that the first year has come and gone so it is time to flower the next year?
Garlic is a good example, I throw mine in again usually Nov, seen posts of planting Oct and Sept. There is an old saying about garlic: Plant on the shortest day and harvest on the longest. Shortest day being around Dec 22.
I have started to make a point of not planting anything considered overwintering until late Nov or Dec. The OW onions I have have 12-15 inches of solid green growth and not a sign of any going to flower. The garlics are much bigger.
Sutty made a good point, many have given up on red onions as the success rate is often poor. I tried twice and do no longer bother. The twice I tried I also found that some grew, some didn't, some rotted and some went to flower - they were not an OW variety, but the results were much the same.
Other factor is stress, it has been pretty dry recently, well here anyway and postings of drought, so some could have been stressed from that and then they will go straight to the flower stalk stage.
You will get a range of reactions out of the onions so not all will do the same thing.
Somewhere out there will be a reliable red onion, just seems that we haven't found it yet on public sale.