I wouldn't think LMA decided that on his own, and I don't think it's from Pop. Pop would still be thinking about DDR shooting more 3s, and shrugging when DDR doesn't.
LMA is a big guy, and the big man coach is Mr. Duncan. What LMA is doing is new for him this year, and Mr. Duncan is new on the staff this year. It becomes suggestive. Also, Tim would know, from his playing days, as much as anybody would know, about the kind of pressure it can put on the defense when the opposing big can shoot the 3 effectively.
I don't know, and could be wrong, but the indications point toward Coach Duncan in this case. Perhaps the most telling reason to think that it's Tim instead of Pop is that it's successful.