This is not the correct answer at all. Statistical analysis suggests that 7-7 rikishi record a disproportionate amount of wins on senshuraku.
For most rikishi who have already reached KK or MK before senshuraku, there is normally little or no incentive to go all out and risk injury in the final match. Unless a major promotion or award is on the line, one more victory or defeat will have a minimal impact on their career, but a needless major injury might. For the rikish who is 7-7, it is time to pump some adrenalin and go all out for it. This human dynamic throws some nasty goop in the sterile world of mathematical models. It is not yaocho. It is not ability. It is incentive. There may be an occasional instance of "good old boy" easing up, but this is not the classic yaocho.
The data is correct; it's the interpretation that's flawed. Short of taking some brain scans and identifying the activities in the "I'm gonna play it safe" and "I got my money/favor, time to pay for it" sectors, data alone will not definitively answer the question of whether yaocho plays a part in such kachikoshi.
(although rikishi are supposedly taught that not going all out leads to a greater chance of injury, so if you subscribe to that line of thought, the only reason a rikishi would risk injury by not going all out is if they had some compelling reason to lose...)
The problem is the data goes on to suggest that the rikishi that have won on senshuraku under these circumstances also lose more often than they should when playing the same opponent next. I understand there are incentives for guys to make kachikoshi, but regretfully the data is consistent with a quid pro quo story.
I strongly recommend reading the whole essay Freakonomics chapter. The analysis is less sterile than you might think :P