Same thing happened to me last week while I was complaining about my unconfirmed bitcoin transaction and seeking accelerators to help speed it up. It just disappeared from blockcypher only for it to reappear just like in your scenario and got confirmed the following morning. Hopefully, yours will follow same pattern of being confirmed soon after it reappeared.
So here's my guess: the wallet or miners that don't include the transaction will forward the transaction to some other miner, and so it results it's being sent again.
Or it's being sent again from the wallet? I don't know, I don't know the protocol in so much depth.