It does seem strange for those French provinces to switch to Poland. When I played on, most of France went right to me (granted, it was a very small, spread out Kingdom) but still, it was a Kingdom (and it's Good to be the King). Only those who were still at war with the (former) King went independent, but then I didn't take his lands, and since they were still at war, they didn't pledge to anyone.
I didn't even need to DOW anyone to get it started, about 6 months later, one of my vassals went to war with Aquitaine, so i joined in and vassalized him. And since he was stil at war with the old King, I then joined in on that fight. Vassalizing him let me force-vassalize the rest who were still fighting the old King. That only left 4 independent Counts, and once my rep got to honorable, they all asked to be let back in the Realm.