A championship is easier to win at LSU. I don't see what's so unbelievable about it.
I don't think anyone is really questioning the "why" so much as the "how" and "when" this went down.
-It's absolutely 100% easier to win at LSU than it is at Notre Dame. Better athletes, less academic restrictions, and lately a stronger profile of putting guys in the NFL. Not to mention the virtually unlimited resources at your disposal...not that ND didn't have that to a degree, but LSU and the SEC are just different.
-Less than 1 week before deciding to leave, he claimed he was never leaving Notre Dame unless he was offered a ridiculous salary in the range of $250M.
-Notre Dame could literally make the College Football Playoff! If Georgia beats Alabama and either Oklahoma State or Cincinnati lose, Notre Dame is right there. I think that's what people are so confused as to why he's doing this at this time.
-Given the way he left Cincinnati, he has a reputation for being an a-hole. Also there is a story that he made assistant coaches (now current NFL coaches) Robert Saleh and Matt LaFleur to shovel snow and park cars for a party at his house.