This goes onto the lack of football situation awareness. Backed up deep inside your own field with 3rd & 20, that's a big up hill situation. The defense will give up the underneath but nothing deep. Plus you know those defensive front guys will be coming with a full rush. Kaep shouldn't take that long to look deep. He only had 2 WR going deep on the outside.
He should have taken a quick look to the outside then quickly throw to the underneath. Chances are good you are not going to get all that 20yards on a 3rd down play. Make the easy underneath the completion then punt. It's not popular, but it saved what happened. Kaep tried to do too much with no ball security and it resulted in a TD for the Chargers.