I don't much about programming and but it looks like your API key was somehow compromised. If someone gets your API key then it is possible they can program a bot to automatically do this. I don't think it is just Bittrex but can maybe happen on all exchanges as they also allow bot trading.
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Wow i dint know they could actually withdraw..
Go to settings and under API key you can see whether you have an API key set. If you have set a API key then there a set of numbers should be seen otherwise no keys should be seen. This how I think your account has been compromised. I may be wrong. Will be good to know if you have or not have have a API key and whether you set this key yourself.