I replay TLJ and the game hints that Tobias was killed by Vanguard agents. I think that it is quite possible that Tobias was a serious target for Vanguard.
Yeah, but there's also the strange part I've bolded
April: Is Vestrum Tobias around?
Minstrum Yerin: Oh, goodness, you do not know? I'm sorry to have to tell you this, but Tobias....he's dead. Murdered.
April: No! By whom?
Minstrum Yerin: We don't know. but we suspect Vanguard agents, of course. In the chaos of the evacuation, the soldiers had their hands full making sure no one was trampled, and that looters were promptly apprehended. When we visited the temple after everyone had left -- there were so many books and valuable artifacts we wanted to bring here, for safe keeping -- we found Tobias by the murals he loved so dearly. He's been stabbed in the heart, and from the looks of it, he didn't resist much at all. He probably even knew his assailant.
April: Why Tobias? He was the kindest, gentlest man I've ever known. Always helpful, always ready to listen. I'll really miss him.
Don't forget that the Guardian's realm was displayed as somewhere in/beyond space (wormhole ftw). The collapse could have been similar to a sun exploding, which would explain why Space travel doesn't work anymore in DF.
Didn't we have confirmation that the problem with space travel in DF was that FTL travel stopped working?
I also don't quite see why what happened in the Guardian's Realm should look like a sun exploding. There didn't seem to be anything that energetic going on.