Hard to say, they might have become disorientated from the pesticide or something else happened. Drowning doesn't fit the MO of the three viral diseases that are known to affect raccoons, canine distemper, feline distemper, and rabies.

Is it possible that they may have been chased into the water by coyotes? If so the coyotes may have tried to wait them out and they simple tired and drowned after swimming around the pond for several hours. Years ago at my dad's old pond his dog chased a young raccoon into the pond at dusk, it tired and drowned trying to swim through the eurasian milfoil to the other side. My dad saw it happen and brought the dog in shortly after raccoon went into the water, he found the dead raccoon the next morning. All it had to do was make it through about 80 yards of milfoil growing to the surface or come back to the spot where it went in and it was home free.