The game has a really high chance to crash(i.e. kill the forhonor.exe process and display error message) after pressing ALT+TAB. I always switch to browsing while matchmaking so this is a really annoying issue. I've sent some crash reports and maybe you can link this post with the crash reports to associate them with the alt+tab use case.
Error text: "The game ran out of system memory. Please make sure to have at least 3GB of free RAM available before running For Honor."
I have 12GB RAM and one second before the crash the usage didn't pass the 6GB RAM mark.
Configuration:
WIndows 10 PRO
12GB DDR3
GTX 780