Blinding Vale 1st pull

Hey, so I joined a BV +13 today, tank says don't touch anything before he gets aggro, to which I reply " sure thing boss", 15s into the key I press my first attacking spell : A measly cast of one Overpower, and I instantly get one shot by a caster spell from a completely different mob than the one I hit (Did 0 cleave or AoE, just a ST spell on a random mob and then poof). I am obviously feeling like the biggest moron for saying I'd watch aggro into the pull only to instantly die.

But I fail to understand how or why I got targeted and casted on by that mob as I did not body pull nor did I actually hit the caster that killed me. From the log can anyone tell me what I should have done differently ? AFAIK the spell is non interruptible and a tank buster, I'm genuinely flabbergasted as to why that mob decided to shit on me 5s after we popped lust when I didn't even gaze at it let alone land a hit on it, I could not possibly have more threat on him than our tank at that point ...

I did see the tank (Protpal) had multiple defensives rolling, was he immune to the spell so the mob decided to take a dump on me instead ?

https://www.warcraftlogs.com/reports/rhfQYntAHTBXNbgd?fight=last&type=deaths&source=3&death=1 (Death event)

https://www.warcraftlogs.com/reports/rhfQYntAHTBXNbgd?fight=last&type=damage-done&source=3&ability=-7384 (My damage timeline)

https://www.warcraftlogs.com/reports/rhfQYntAHTBXNbgd?fight=last&type=damage-done&source=142 (Tanks damage timeline)