I think the top players simply run out of possible targets, so the lower league players are being targeted.
The way it is now, every time someone PVPs, the total amount of trophies in the whole system decreases, as the defenders loss is always bigger than the winner gain. If the attackers win, he gains 20, and the defender lost 30. So a net 10 is lost from the system. If the attacker loss, he lost 40, the defender gain 30. So a total of 10 is lost from the system as well.
It is simple math, so eventually everyone will be at 0 trophies, unless someone can get negative trophies?
The top players may keep their amount high by always winning. But their target keep having lower trophies, thus keep dropping from L4 to Gold 1, to gold 2, to gold 3, etc. I would not be surprised if soon, even top 500 players can no longer have more than 4000, thus be dropped from L3 to Gold 1. There is simply no way to stabilize the system, and everyone will be at newbie league soon, maybe in season 11.