Does Mojang occasionally invalidate all sessions, making it so everyone has to restart Minecraft?
I've been playing Minecraft for a few years. I know if you log in with another instance of Minecraft, your session will be invalidated. However, when joining a new server after a few hours, I will occasionally have to restart Minecraft after being kicked for the "Invalid Session: Try restarting your game" without actually having logged in on a separate instance.
It's happened a couple times in the last few months, but nothing else suspicious is going on.