Why the PCA for these RCA congregations?
That's a big question. First of all, one might think about what life in the RCA for them was like earlier and what it had become. Life earlier in the RCA for these congregations (the ones now becoming PCA) had been such that they were largely left to operate as they saw fit with respect to a whole range of issues, even though they were clearly more confessional and conservative than the administrative offices of the church with respect to these issues. The RCA has been for some time fairly loose in its centralized governance. What it had become in recent years, however, was more demanding with respect to all of its member churches, particularly regarding women in office and the like. In other words, greater conformity to certain liberalized positions has come to be required throughout the whole church. Conservative congregations are no longer being afforded the luxury of maintaining their conservatism and not having to "tow the line" put forth by the church's leadership. So it has become evident to these departing congregations that they would not be able to resist the liberal demands any longer.
Where to go? What would be ideal for them would be another Reformed body that would permit them to be confessionally Reformed but with a bit of a slack hand as they were accustomed to earlier in the RCA (before the demanding liberal hammer fell). Who would this be? What body in NAPARC qualifies? It would not be the URCNA, as that would be perceived by most of them as narrower than they would want (whether rightly or wrongly perceived is another matter and a rather complicated one). Still, then, to which NAPARC body should they go? I'll not proceed through all the NAPARC bodies in turn, but they perceived that the body that would be Reformed in doctrine (and allow them to be so) and allow them the freedom that they want as congregations would be the PCA, which has a grass-roots feel to it and allows local churches considerable discretion.
Much more could be said about this, but it's probably not necessary for those who know the NAPARC bodies well and for those who don't it would require more than I am willing to say here.
Peace,
Alan