First of all, welcome to the forum, Essentialy, and secondly, I feel really sorry for what you've suffered. Using a HW is way more than most people do to store their coins safely, and it must be really hard to realise that although following the right steps, some money you counted on simply disappeared.

I'm afraid that I can't help much as my technical knowledge is less than basic, but from other cases I read in the past the most probable answer is that your pc was already infected when you created the seed. Otherwise, it could be because someone got access to your keys, but if you wrote it and immediately stored in a safe, and you're sire that no one has had access to that safe, a very improbable, yet not impossible theory, could be that your phone or webcam were compromised (quite an incredible theory, but I've read about it more than once in the past).

I hope you get more help from other users with better knowledge and you get your coins back, or at least know where they ended.