Here's what PFF's Ben Stockwell had to say on the accolade:
In a close battle at the top it is, however, an "undersized" defensive tackle who takes the crown as the league’s best defensive rookie in 2014. There were questions over how Aaron Donald would cope with the physicality and the grind in the NFL trenches during the pre-draft process but Donald turned those questions on their head in his rookie season. At the end of his rookie season Donald had posed more questions as to how NFL offensive lines could cope with his blend of speed, technique and agility than he had to answer about durability or power. In the second half of the season he earned a higher grade and played more snaps than he did in his first eight weeks, breaking through rather than hitting the rookie wall.
Much more than just a one-dimensional upfield pass rusher, Donald was able to use his strength and leverage to be a destructive run defender as well. In fact, his form as a run defender really came well before his late surge as a pass rusher (20 of his 44 pressures came in the last five weeks). That balance sets Donald’s inaugural season aside as a special one not just in this year but in any. He wasn’t just an impact player racking up the big plays but flawed in other areas, right away Donald brought out his A-game and was among the league’s very best defensive tackles based solely on their play in 2014. What might have been taken as a luxury pick for the Rams in May proved to be one of the best defensive linemen in the league in 2014, making Donald a more-than-worthy winner of our Defensive Rookie of the Year award.
Hell to the yeah.