It happened to me this past cycle. The e-mail secondary invite came first, then the e-mail interview invite. You still have to submit the secondary though before the admissions committee will make a final decision on you. Also, Pitt is non-rolling, so you get your decision with everybody else at the beginning of March.
I don't know what the exact correlation is between this pre-sec invite and chance of acceptance, but I was accepted with the first batch this past March and am matriculating next month.
Good luck!