Brandon Morrow of the Toronto Blue Jays generated a strange pitching line this season. He strikes out a ton of batters, 10.4 per nine innings, and that usually leads to a low number of hits allowed. The league is hitting .244 against him. That's not great, but it seems a bit high for a pitcher with that many strikeouts. Of the top ten pitchers in the major leagues in strikeout percentage, only two allow batting averages over .240 (Zack Greinke the other), and most are below .220. Brandon's walks are a bit high at 3.4 per nine innings and his home runs are about average. Given his stats, Morrow looks like a pitcher that should own an ERA in the mid-3.00s, not one over 5.00.
The problem with Morrow comes with men on base. This is best demonstrated against left-handed batters, but it applies to right-handed batters as well. With the bases empty, Morrow keeps the ball away from lefties:
That heat map generates a .200/.283/.285 slash line. In 223 left-handed batters faced with the bases empty, he allowed 10 extra-base hits.
For some reason, Morrow abandons his successful location and comes inside to lefties. This results in a .266/.323/.475 slash line. In 158 PA in this situation, Morrow allowed 18 extra-base hits! The effectiveness of extra base hits comes their ability to advance runners a long distance, especially in scoring runners from first base. Morrow changes his location at exactly the wrong time and gets hammered.