Last year's Superstar Shakeup came just 265 days after WWE had already split the roster with another Draft in 2016. In that eight months (and change), the powers that be decided things had grown stale on both shows. If they thought that then, imagine what they're thinking now.
Both Raw and SmackDown need some fresh faces, otherwise WWE's creative team will be forced to put on the same tired matches we've already seen for the rest of the year. The need for some stars to switch brands isn't only brought on by the need for new opponents though. No - there are those also in dire need of a heel/babyface turns to freshen them up full stop.
A change of scenery would set the scene for that, and there's no better time for WWE to pull the trigger than post-WrestleMania 34. Once the biggest show of the year is out of the way, the company will be looking for something to ease that traditional lull after 'Mania anyway.
Presenting something new on both brands and rejuvenating some stagnant careers would be a start...