BTC balance partially unavailable whilst outbound payment was pending

I sent a small BTC payment via BitPay of around $30 equivalent yesterday which took around 19 hours to finally confirm. During this time, over half my balance in BitPay was unavailable to spend such that if I'd wanted to send a second payment with the rest of balance, I wouldn't have been able to. Once the outbound payment confirmed, the full balance instantly became available again. Why did the pending outbound payment lock up over half my balance like this? (I had no incoming payments pending). Thank you all.