I've seen this once before. Because onions are a bulb, just like alot of flowers if they are left in the ground they'll come back again. It sounds like your onion is gearing up to flower. If you pulled one up I'd bet the bulb would be small compared to a regular onion. I have a few from a friend, using them as green onions, the stalks are crazy big huh? Or you could wait and let it flower, might be pretty.

I planted some last fall to and they overwintered but the tops did NOT die.
Since they were packed closely, I took them all out and transplanted
at proper spacing. Now they are growing fine and no flower buds on them.
I have also planted some from small onion sets. Some of them bud but I pich those off as soon as I see them.
Those onions that go for seed, are not keepers and should be used before the rot inside.