There are two potential causes here - either positive ions or bacterial activity.
Soy curdles easily in the presence of positive ions, that's the basic principle behind making tofu. I don't see many potential sources in your recipe, so if this is the case, my guess would be that there was some combination of hard water, possibly high-ish magnesium levels in the soy itself, and maybe some other deviations in the soy chemistry (not every cultivar or every batch of the same plant is equal chemically, and if you hit the right - or in this case wrong - combination of compound levels, curdling occurs easier).
The other possibility is that you had enough bacterial activity that it produced acid which curdled the soy proteins. While 3 days in the fridge and 3 hours on the counter are considered safe, there can be one-off cases where you get spoilage going on. This is much more typical for animal milks though (that's how you get buttermilk), I'm not sure how likely it is to happen for soy since I have never observed firsthand how it ferments.
You should be able to make the distinction. If you have off-smells, sour or acrid taste, or other signs of fermentation, this is likely bacterial activity. If the whole thing is bland and firm-ish and reminds you of tofu, it is likely a chemical curdling.