When you spend your coin, you are exposing your public key. On average, there will be 5 minutes between when you broadcast your transaction and when the next block is found, so an attacker with a sufficiently strong quantum computer will have 5 minutes to calculate your private key, and double-spend your tx after you broadcast your transaction.

There are threads about this elsewhere in this sub, and I have posted about why QC is unlikely to be a threat to bitcoin. In short, the value someone can get from using QC to calculate bitcoin private keys is less than the value of keeping the existence of this technology secret.