2 questions for you (3 really, but the last two are almost the same thing):
1) Who was the pitcher? If it was somebody with a low BB/9, you probably fell victim to the psuedo-randomness (I like to call it "error radius") that the game imposes upon pitchers with those ratings. If you've ever played around with Pulse pitching you can probably understand what I'm getting at here. How do you propose that the game accurately reflect that lower stat, if not from what I just described? This game is not all about perfect user inputs (nor should it be IMO) - the realistic stats and outcomes have to come from somewhere.
2 and 3) Regardless of who the pitcher was, what were his confidence levels both overall and for that individual pitch you threw? If one or especially both was low, that error radius I mentioned above increases and you're more likely to throw a fat pitch (or a wild one) even if you were aiming to the corners.
It's happened to me a lot of times myself. But I shrug it off, because it really does work both ways. And you'll understand that if you ever try to aim a pitch far off the plate, but have it end up nipping the corner for a strikeout. It's especially important on HOF and Legend, where the CPU's PCI is enormous and it will not miss your mistakes very often. Pitch cautiously and accept that you might give up some walks. It's better than risking throwing the pitch that you described.