This season he's hit a HR on 3% of his plate appearances. Never before in his career has he homered in that high of a % of his PAs. On top of that, It's been 4.8% while in Chicago, and was 2% in Oakland. His HR/FB rate in Chicago of 19.6% is nearly triple the 7.1% he put up in Oakland in the first half of the season and more than triple his career mark of 5.9%. The league average is supposedly just under 10%, which fits for Andrus being typically less than that since he's not exactly known as a power hitter. His FB rate is even down since he came to Chicago from Oakland, although not outside his career marks, so he's hitting fewer FB and they're just going out of the park.
On top of that, his exit velocity in Chicago is lower than any season of his career since that was tracked, his walk rate is lower than any season of his career, his K rate is higher than any season of his career.
His batting average in Oakland was .237, his OBP was .301. His batting average in Chicago is .277, his OBP is .312. This is a guy who started swinging a little more aggressively, got lucky on a couple swings, and had a surge of HR that he probably hasn't done before and won't duplicate again. Everything about his numbers screams "would have an OPS in the .600s but got a few extra HR swings in Chicago", which is totally a thing that baseball allows over stretches of 200 plate appearances.