I feel like--and of course, this is all speculation--there just aren't many humans out there as long and athletic as Groot, who can also hold the line against OLmen. I wonder if part of his early success is due to the fact that OL just aren't accustomed to his length, and honestly don't know how to handle a guy like Groot.
(also, Miami's OL wasn't very good, to begin with; we'll see how this pans out, over the course of the season, I guess).