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If it's a single specific account, it's possible that other account is compromised and a bot keeps trying to cancel their trades to trick the other person into giving away their stuff.
Otherwise, it could be a bot latched onto your account that's only trying to go after TF2 stuff. You might want to go through the whole 'secure your account' procedures just to be safe.
Scan for malware https://www.malwarebytes.com/
Deauthorize all other devices https://steamhost.cn/twofactor/manage
Change passwords from a clean computer
Revoke the API key https://steamhost.cn/steamcommunity_com/dev/apikey
Generate new backup codes
If you did, this issue is completely normal. This happens because the order of the items is always different in your inventory every time you start a trade (only for items of the same type). That means you might have put one key into two or more different trades. Once a trade is completed, all other trades containing the same key are cancelled. Since there is no way to see the difference between two items from the same type, I would suggest you to not have multiple trades open at once.
This can be the other way around too of course.