PBP definitely think it was Brexit that Sinn Fein targetted them for. PBP took a principled lexit stance.
SF must have been rocked by losing seats in their heartlands last time West Belfast and Derry. It seems they went all out on the doorstep in a similar fashion to what the IWCA must've experienced when making initial inroads and similarly rocking the Labour Party in particular.
If Brexit is all SF had as ammo, that too is interesting and shouldn't lead to despondency for PBP. Also, two elections in rapid time will also hurt minor party's resources. Last time, the Bog was swamped with PBP posters; this time, despite only seeing outskirts of Derry, I saw far fewer posters of PBP. Two weeks back, they along with other minor parties were invited but couldn't get a panellist along to the Nolan live election special.
SF vote up 4%ish, too early for detailed breakdown but speculation that hard brexit since last elections may have led to many ethnic minorities especially east Europeans coming out for SF.
as well as above mentioned UUP transfers helping SDLP eventually, I think I also saw evidence of SDLP helping UUP as well.