I'll just put it down to BioWare's bad AI. I still find it hilarious how AI in ME3MP would walk backwards up the stairs of Firebase White, aiming at the roof, because that's where their "target" is. I didn't really expect much improvement there, almost no AAA studio I can think of seems to put serious time and effort into getting computer AI up to par with the resources they've sunk into environment graphics/physics. Its why I laughed at Ubisoft's claim that AI was the reason of the high minimum requirements on AC:Unity.
There are clearly things wrong with the routines driving the "threat" system when it sends freshly spawned units clear across the map at an inactive player. If I had to take a guess its over weighting on the "distance" of the "back rank". The freshly spawned rogue is "behind" (relative to global path lines) the closets target, likely has a higher DPS weapon, and is "away" from "allies". Depending how the weighting is setup this may be a larger "threat" target than the Warrior who's actively attacking.
Emergent behaviors like this are hard to theory craft in a limited pool of internal testers, especially if you don't sit down first and plot out a fairly good list of possible outlier influences. You really need someone in the room who's joy in life is to think of the most devious ways to break all the toys, and to take that person seriously when they come up with something nonsensical.
Although to be fair, wouldn't you try to bull past the low-damage high-armor "warrior" and get at the squishy high-DPS casters?