It was hilarious, a total 180. And yeah, I liked ALW on AW and liked her at the show initially with BW. It was too bad. But she was placed in a position where she became unbearable fast.
I think it may have been a combination of things, but I'm not sure. I expect Guza liked the idea of finally bringing Scorpio back (he wrote there in his heyday, after all) and he certainly early on in the return pushed this cynical take on the character being much more cavalier about Robin and his family vs. his life as a career secret agent. I know Tony Geary had also pushed for it for years - allegedly, his original version of "Endgame" which he wrote with Irene Suver (and which JFP discarded) had featured Robert's return from the dead.
I think it also benefitted the show at the time. Kimberly McCullough had just returned on contract, was now a major foundational player again and in spite of issues with execution they began to re-orient a massive portion of the show around the hospital again, focusing in on Robin and her love interest Patrick. It also synergized well with the network striking gold in primetime with Grey's Anatomy. To the show's credit, that return to a significant hospital focus and several major on-contract hospital characters at any time has pretty much remained ever since late 2005-early 2006. That came from Robin's return.