You probably have mistakenly exposed your wallet seed phrase somewhere. The way seed phrase is structured, it can't be broken by anyone, not even a hacker can break the special functions it was pass through to create you a seed of 12-24 special words. It's either you didn't back it up well in safe place or you did back it up properly but you didn't keep it in a safe place and another person has seen it and was fast to send it out to another wallet destination.

What you should do next time, if you want a safe and secure wallet, you should use a hardware wallet. It's more secure and safe than the software wallet. Unless you exposed your seed phrase to anyone, it can't be compromise because your keys are actually not connected to the internet. But, it doesn't mean you should misplaced it anywhere unsafe or in unsafe hands, if they know your pin, your coins will be wipe off. Stay guarded next time.