I'm testing my bitcoin marketplace site using testnet. Sometimes I'll simulate a payment from a buyer to a seller. In this scenario, my site generates a payment address and the fake buyer is supposed to send money to it. But before I do anything, a mysterious payment will be sent to the address and cause the payment to be partially or fully paid.
I'm not sure if this a bug with my site or the nature of testnet. Here's one such transaction: https://live.blockcypher.com/btc-testnet/address/miX7hFUHPciQaUC2QR88BMCUaR18aH2857/
I'm not an expert on blockchain but it seems like someone sent 0.68885375 BTC and the change valued at 0.00059542 BTC ended up going to my address instead. Why does this happen? Isn't the change supposed to go back to the sender?