If you're tank is doing their job, you actually shouldn't have aggro to need to run out of cover
Blame them! Its always the tanks fault! hehehe.
Simply not true.
The moves he is mentioning (such as the rock throw from foreman crusher) have absolutely nothing to do with a tank doing his job. This is an operations level boss who attacks multiple people in a variety of ways all at once.
The rock throw in particular is a 100% random mechanic, the boss can be full agro and beating on the tank (with his fists and frontal attacks) and the rocks he tosses are everywhere else on the map in random places.
90% of the operations bosses have attacks like this which randomly target other places and people in addition to their main attacks. The tank could be 100% perfect and the bosses in operations are still going to hit everyone else with their secondary attacks because that is how operations bosses work.
Whether its the rocks, the missiles, the lightning balls or a million other things the operations bosses use, they all attack the group as a whole with something; you always need to move, you always need to be ready to move. If you don't know this, you don't even know the level of bosses he is commenting on.
Typically these attacks are area effects with big obvious visuals for you to physically dodge out of the way (a target reticule on the ground that you need to get out of, a flashy bomb on your shoulder that you need to move away from everyone else, a lightning ball that you need to go pop without dragging it into other people.... etc... I could go on... it's nearly every boss in operations...)
In every case a sniper is at a disadvantage with bosses that require movement and response from every member of the team and those mechanics are present in most end game content. Every person must be capable of moving instantly or risk losing the encounter for everyone, and yes when its a close fight, every second counts. Snipers lose half of the possible response time in virtually every operations boss fight simply by having to stand up to move. When seconds matter, it adds up, and having a sniper die (or any of your DPS) puts you in a very likely position of hitting the rage timer and losing entirely.
And when those attacks hit for 5 to 10k, its not even about having gear, those fights are won by everyone being able to move and dodge instantly while maintaining whatever else their job is (DPS, heals, keeping agro on main attacks, etc)