Had Rossi pitted earlier, Lorenzo would have followed him in and they both would have left the pits at the same time on slicks. Because Rossi stayed out, Lorenzo pitted and pushed too hard on the slicks trying to quickly catch Rossi. Rossi's "bad decision" to stay out on the wets led to Lorenzo's bad decision to try to make up too much time too early once he got back on slicks and ultimately gave Rossi a bigger points lead.
That's a bit of a stretch, and not how Rossi or Lorenzo saw it either. They both said it was a mistake to stay out. Good analysis from David Emmett at MotoMatters:
https://motomatters.com/analysis/2015/09/13/2015_misano_motogp_sunday_round_up_good_.html