My personal theory; no matter how much they might have sucked in a game offensively the awareness that they can explode at any moment wears on you mentally. So many teams get leads on them and then seem to play not to lose; they start nibbling at the plate not wanting to let the offense get going, which leads to walks and/or wild pitches, which gets guys on base then you tense up and nibble more until you finally make the mistake that they make you pay for.