McCormick, a junior, entered the season averaging just 3.7 points per game and scored 12 points in Western Michigan's first two contests of the year. He finished 9 for 11 from 3-point range.
Trailing 45-41 early in the second half, the Broncos went on a 40-24 spurt to seize control and build an 81-69 lead on Tucker Haymond's layup with 3:09 to play.
The Broncos held a 31-30 lead at halftime.
Thomas Wilder added 23 points for Western Michigan (2-2), shooting 7 for 10 from the field and 7 of 9 at the free throw line. Haymond finished with 11 points.
Justin Nafso led Rochester with 23 points, seven rebounds and seven assists. Milton Cribbs added 18 points and Jaylen Larry had 15 for the NAIA Warriors.