It happened multiple times to me too. Sometimes when I go past the mirror the traps are already firing in there and the book is gone, but I haven't been able to understand what causes it.
In one case I went past the mirror, then left to recover the key to the book and when I came back it was gone, with traps firing. I just figured I should have picked it up right away.
In another case I got past the mirror, saved and quit the game. When I resumed playing, traps were firing (they weren't when I saved).
Last time I was there I killed the undead in the caskets and when I went past the mirror, the traps were firing. I reloaded, ignored the caskets, went past the mirror, recovered the book and then dispatched the undead.
I'm pretty sure in none of these occasions I had a quazit with me.
