This post is relevant… What BTC sellers on the Counterparty Distributed Exchange need to know?
Basically… BTC is not used on the DEX very often because it requires some additional steps when using BTC that you don’t need to take with other assets.
1.) User A lists their order to sell NVST for BTC
2.) User B lists their order to buy NVST for BTC
3.) Users A&B have their orders ‘match’ on the DEX
4.) User B now has 20 blocks to pay the BTC via a BTCpay transaction, otherwise the order expires.
Most likely this is what happened… no BTCpay transaction took place, so the order expired.