Really interesting data set; thanks.
There really isn't a good negative correlation with slider percentage and wOBA. A better case can be made if you only are looking at slider percentages above 25%, but there you run the risk of using a post facto test. There was no reason to choose a slider% of 25% prior to looking at the data, but if you want to show a negative correlation between wOBA and slider%, it serves the purpose. The problem with any post facto test is that you can easily mislead yourself, and it isn't really possible to do any statistical analysis on the data set. When you start pulling correlations out like that, it's possible to find all sorts of spurious correlations.
Just looking at the data, the negative correlation between a slider% of 25% and wOBA only really works for the beginning of the year and then over a very small sample very recently. There is also a small sample (and that sample is actually bigger than the recent sample) where slider% is above 25% and Swanson's wOBA is fairly high. So, really, there may be a negative correlation between wOBA and slider% above 25%, but it depends on that data from games early in the season. After that, it falls apart with as much positive correlation as negative correlation. I admit that it is an interesting possibility, and it's worth keeping an eye on, but there needs to be more data. It could easily be what we call "fairy gold". It looks great in the first light of morning, but then once the sun shines on it for a while, it all goes away.