The chain of events started with the trade of three mid-level prospects—Robbie Grossman, Rudy Owens, and Colton Cain—to the Houston Astros for Wandy Rodriguez, an upper-middle rotation pitcher. With the Pirates today, he might only be their fourth starter—although he would have been their second, two years ago.
The Pirates already have a surplus of pitchers. The trade for Rodriguez caused a chain reaction, sending Kevin Correia to the bullpen, where Brad Lincoln is already, and pushing Evan Meek back to the minors.
Correia is ending his two-year signing period, which is to say that he is now a two month rental. It would be nice if the Pirates could trade him and, say, a mid-level prospect for someone like Hunter Pence, but that's not too likely.
According to FanGraphs, Correia's been a replacement level pitcher for most of the year. Thus, he's not now worth the $4 million a year he's being paid (although he served his turn last year during the Pirates' "miracle run" for two-thirds of a season).
A more acceptable consideration in a trade for Pence would be Brad Lincoln, who has been worth 0.6 wins over replacement (WAR) to the Pirates, to date—he would also be under club control until 2018. Hunter Pence has been worth 1.2 WAR (These WAR statistics are also courtesy of FanGraphs).
The Phillies want a good middle reliever like Lincoln. They'll probably want just a little more for Pence. That "little more" could be another mid-level prospect. Or it could be the second pick they won in the draft lottery, which can be traded for the first time, and might yield a prospect.
So Rodriguez takes Correia's place in the rotation, Correia fills Lincoln's shoes, and Lincoln can be traded for Hunter Pence (or someone like him). Pence AND Rodriguez would represent a nice return for the prospects (and Lincoln) that the Pirates gave up. By trading for Rodriguez, Neal Huntingdon at least created that possibility.