The speculation goes something like this. Porcello was originally a pitcher that relied heavily on his 4 seamer. Being called up at such a young age, he was having trouble locating the 4 seamer, and Detroit felt he would be better off throwing more 2 seamers, which he could command better. So, Porcello became a sinkerball pitcher. He had some success with that for a few years, but he did not have a very high strike out rate.
Detroit, and subsequently Boston, thought that he could be an even better pitcher if he went back to his roots and started throwing his 4 seamer more to improve his K rate. His K rate is up, but so are his hard hit balls.
I don't know why he has been unable to locate that fastball consistently. I also don't know if this is the correct approach or not, but I guess time will tell. Also, this is just speculation from some writers and commenters, and has not been confirmed by anyone in the know.
IMO, if he was successful as a groundball pitcher, I don't see the need to improve his K rate, particularly if it's going to take away what used to be his strength.