the NPC AI is pretty simple and it pretty much plays its cards in order, so when Return comes up it's just because it was that card's turn to be played.
In the Lumious example, it often plays Exploding spores then Burning Sun for a field wipe. If it didn't play Burning sun next turn, then he either didn't have enough SP, or maybe the grim had been baned.
It's just an NPC, it can only handle simple commands, so don't think it's trolling when it plays rediculous moves. They just don't know any better
[shameless link] I can still claim I'm a free player cause I haven't spent any of my
money on the game, but instead I used Prizerebel to get my EX packs. Gran Train = viewtopic.php?f=13&t=11761