Scripters cause lag by running scripts in 3rd party programs. These don't even have to be the egregious ones with 8+ characters all moving together. This can be someone playing a single character that is just running some automation or auto targeting crap. There are probably a fair amount of these scripts that are just super inefficient. Ever attend an EM event and a scripter dies and they just spam the crap out of Enemy of One and it fills the screen with messages? Because their script is so dumb, all it sees is "Enemy of One is off, try to turn on" and its just a spam fest. Every one of those commands the script is sending goes to the server and causes lag.
They also didn't say the corpses are causing the lag, but a by-product of the fix is that non-paragon corpses will be deleted. This tells me it is something happening when a mob dies that is causing lag, and on grinders like Atlantic where there are hundreds of mobs dying every few seconds, I imagine it adds up.
I will just say, if you are looting the trash mobs during these events, you are doing it wrong. I am noticing a trend between posters here and on the official forums that are complaining about not being able to loot the trash mobs and getting low event drops. If you are stopping to loot non-paragon mobs, you are going to be killing less things. Event drops are going to sell for a couple mil each through the whole event. You are better off just selling those than trying to make money by looting corpses. It's also just these event mobs, its not like they turned off corpse looting globally.
Those saying this is an Atlantic only issue -- its not. There were players from Japanese shards complaining about similar lag, and if this ToT follows the trends of every other ToT event, the players will eventually rotate through all the shards because they want to get drops for all their cross-shard characters or to sell on all shards. It may not be a problem now 2 days into the event, but this change is future proofing the issue for all shards.
Players wanted town invasions again, we got town invasions again. This format is much better than the previous titan invasions, where everyone scrambled to get the mage generals first for the spellbooks and then left, leaving a handful of people to clean up the trash no one wanted to kill just so the town would end and move on to another town.