They come in bunches
The Cubs missed Biggio 20 times last night. It's been over a week since his last HBP, and it may look like he's falling off the record pace but the projection in the side bar of this page is slightly deceptive. It assumes he'll keep getting hit at regular intervals throughout the year, but historically Biggio tends to get hit in bunches. 146 of his plunks have come within 3 days of another plunk, and he's gotten drilled twice in the same game on 19 occasions.
Craig Biggio has only been hit twice on May 1st, by Pedro Martinez in 1997 at Montreal, and by Britt Reames in a home game in 2002. Both games were losses to the Expos.
Mark Prior has never hit Biggio, has no hit batsmen this year, and only 19 in his 73 career starts.