It was because he started slowing down, then the fielder bobbled it, and then he sped up (but way too late). There's a 99.9999% chance he's out by a few steps either way, but he was booed for the optics of the effort
Cubs will lose 1-0. Boyd will pitch 8 IP of scoreless ball. Palencia comes in, and the Brewers score on a catcher's interference, wild pitch, and two consecutive balks.