She claimed to come back because Toby (Keegan Allen) came to talk to her (though how exactly he found her remains unclear), but we're not entirely sure that story makes sense. She thought she knew "A" was Mona (Janel Parrish), but she was scared into not telling because Mona threatened her son, but then she decided to come back anyway because of Toby's persuasive powers?
Listen, we get that Mona is unhinged and a bit scary, but Doctor Sullivan is a grown woman with lots of experience working with less than stable patients. We find it a little hard to swallow that she was so easily swayed one way and then the other by teens.
Here's our thought: Maybe Doctor Sullivan is working with Mona (either voluntarily or, yes, maybe because of blackmail). The only explanation the girls have about Mona's mental state is from Doctor Sullivan, so if she's lying about anything, that could cause major trouble down the line, and in Rosewood if something can cause trouble, it normally does. This would also explain how someone managed to casually visit Mona is the psych ward, which we imagine is not actually an easy thing to do.
On the other hand, maybe we're still on a finale high and are reading too much into things. What do you think?