A think the "Week of Their Career" theory is the most likely culprit. Though I can see how taking a few weeks/months to get all the off-course benefits (media appearances, speaking engagements, corporate gigs) of a major championship could be really distracting and sap any swing or short-game mojo.
Or maybe some are guilty of celebrating that one win way too much...for WAY too long. And I imagine a relatively minor injury can have a disproportionately large impact on one's golf game at that level.
In a couple of cases (Simpson, Ogilvy, Leonard), their titles literally fell into their laps from others' collapses (Furyk at Olympic, Mickelson famously at Winged Foot, and Van der Velde, respectively), so I wonder how much respect those guys' garner even among their colleagues. Okay, Leonard nailed that unbelievable putt at Brookline, but still.
How Louis Oosthuizen hasn't won more with that swing is nearly incomprehensible.