I think that they just get a bit too complacent + Mike tries to give Nash and Kobe rest in such big leads. I watched it happen many times this season. If Lakers start poorly, Mike usually still waits to bring them back. He does it with 6 - 7 minutes to go and if Lakers played badly before, they all stop playing all together, just waiting on Kobe to bail them out.
Rival teams understand that as well and focus on him even more than usually.
I think that Mike needs to change a bit the rotation in the 4th quarter. Maybe pull out Kobe with 3 - 4 minutes left in the third and bring him back 10 minutes to the end or something like that.