I don't really remember, but did Lexie and Nicole ever have run-ins with one another in the past? Like when Lexie slept with Brandon or when it came out that Nicole's mom and Abe had a past together?
I honestly can't remember any confrontations with them but that could explain the hostility if that had happened. Then again... this show has completely forgotten about Lexie and Sami's hatred of each other so... that's probably giving them way too much credit if that is even an explanation.
These are some of the few scenes I remember Lexie and Nicole having during Nicole's first time on the show.