What you saw was probably the effect when Shattered Defenses were up.
Falcosaur Swarm leaves a debuff called Shattered Defenses, and during that the target takes double damage.
420 * 2 = 840 (below 25%)
Double that with Shattered Defenses = 840 * 2 = 1680
And if you crit as well … 1680 * 1.5 = 2520
Which doesn’t get me to your 2840, but I haven’t had coffee yet, which means I may have missed something, or perhaps there was something else in effect in your case.
Shattered Defenses is being nerfed a bit with pre-patch; it will yield only a factor of 150% rather than 200%, and it will last for only one round instead of two. However, it will still be very powerful.
See also Chrominius Howl/Surge of Power and Ikky Black Claw/Flock.
P.S. Oh, maybe your 2840 was with one of the other Falcosaurs with more power.
It is by coffee alone I set my mind in motion.
It is by the juice of the bean that thoughts acquire speed,
the fingers acquire shakes.
The shakes become a warning.
It is by coffee alone I set my mind in motion.