Why could be a hundred different reasons, unfortunately (with some grows I witnessed seed, there was no discernible reason...). At the end of the day though, lets just call it stress (Heat, light, nutrient, underwater, overwater etc... etc...). That is a male organ, but you caught it, good eye👍. I like to look at the positive side of things and would like to say it ain't over yet and only time will tell. From the pictures of your bud sites, I don't see any seeds pushing through the bud or forming new male organs. This might change, but for now, let's stay positive👊. This is a good sign that you've only found one and it doesn't appear to be fully seeded at all.
Recommended course of action: Full inspections daily, top to bottom (I bet hard cash you're doing this now already). Any new male seeds or nanners pop up, pluck them off and hope for the best. This is a tough spot to be in and I'm sorry that you're experiencing this. It does happen to even the most veteran growers (Just talked to one yesterday. He lost three 😞to a full seeding...but you're not fully seeded and there's hope👍). I wish you the best of luck in the coming days. I hope this was isolated and everything will be just dandy. Cheers my friend. Stray frosty.