2.1 M crit is much more than 5 UAs, most likely it was the ST build revolving around contagion, soul harvest , conduit and the pvp talent rot and decay.
basically pops cds at 5 shards and dumps all UAs possible into the target then drain life to keep the stacks from falling off while constantly adding more UAs into that huge stacked UA whenever a shard procs, at some point you can reach around 12-15 UA stacks and dispelling that means death , yes.
If you let the warlock cast that many UAs AND you're not even aware of what UA does and end up dispelling it , that's entirely your fault.
On a side note, MW monks pvp talent healing sphere can remove UA without getting hit by the backlash, it is quite frustrating for the warlock.