This has been a fall from grace batting year for Mookie, so I decided to look for other top hitters and who also had big dropoff seasons. There aren't many. Just looking at batting average, and considering their prime years only (of course people drop off after 30 or so). I didn't look at anyone before the "Ruth years", and I didn't spend all day at this. Couldn't find a way to search for this, so this was "by hand". I therefore most likely missed some cases. For comparison sake, Mookie has had a .318 to .264 in '16-'17 and a .346 to .262 year to date last year, this year.
Babe Ruth dropped from .378 in 1924 to .290 in 1925. I believe that was when he was maxing out on the booze and food binging (and carousing?). He of course straightened it around from 1926 on. Pretty big time.
Bryce Harper had the great season with all the numbers in 2015, then dropped from .330 to .243. He had a bounceback year in 2017 but has fallen back again the last 2 years.
Rickey Henderson had his ups and downs hitting wise, a, 319 to .267, a .324 to .263 and a .325 to .268, but was he a great hitter anyway? Not really, he was a great on base guy and base stealer.
Frank Robinson had a .342 to .258 dropoff in '62-'63 but his games were down so maybe he was hurt.
Yaz had a .329 to .254 in '70 - '71. He changed his stance a few times and mostly righted the ship. The .254 was in his age 31 year, so maybe he was starting the decline anyway.
Ripken had a .323 to .251 in years 30 and 31.
Adrian Beltre had a .334 with great power numbers and dropped off to .255 with not so great power nos. age 25-26. That started 5 down (for him) years hitting wise, then he came back and hit over .300 for a few years in his 30s.
Frank Thomas had a .347 to .265 at ages 29 and 30, bounced back.
What Mookie is in the midst of is pretty rare. It's not just one off year after a great year, hitting wise, but two. Somebody mentioned his size. He does like to hit for power, so maybe he has to max out on his stride and swing from the heels all the time to produce the power given his size. Any injury or anything else that cause variance in the swing or stride, inside/outside might throw him off. He could use the best hitting coach in the game right now. Who is that?