There's 2 possibilities to this
1. Sabo saw Imu and somehow was able to see which island Imu was going to nuke, he went there to save it while the other revs went on ahead, he evacuates everyone on that island before the nuke
2. Sabo saw Imu and Imu wants to kill him now, sabo got separated from the other revs during their battle against the 2 admirals, he ended up at luciula kingdom, and contacted dragon. Marines intercept the call and they know where sabo is. Imu gets this info and nukes the island as he wanted to kill sabo.
I think the second one is more likely