Wow that really sucks. These things can happen, nobody can see it coming. Even if people here say that they have special security to deal with avoiding situations like this, nobody is really safe.

This can happen to anyone at any time. There is no 100% way to avoid getting scammed, hacked or robbed. It is just part of the human condition.

If I were to guess, I would say you either gave away your seed phrase without knowing it and/or you had malicious software on your device. But you have not really given us much information, so there is nothing to go on.