Is random failure to rise explainable?

I've been using lately a starter that I started about 5-6 weeks ago. Local wild yeast.  It has performed fine.  I've learned to make it less sour, how to feed and make it happy etc.  So, I made a pretty typical multigrain large loaf (about 2.5 lbs) with a 1/2 cup of the starter like normal.  Do this 3 times per week, sometimes 4.  I set it as usual to rise overnight (there's some prior steps that I've long adopted, using a 100% hydration flour-water soak seperately from the 90% sponge, mix it all up, a fold or two, depending on what I've added, all I think is supefluous to the question I have).  It is typically around 65-70% hydration when I have calculated (it seems to peform better at ~70). It rose about half it's usual and just stopped.  I decided to leave it in the morning to after work, and it hadn't done anything.  I decided I better get it going, so added a tsp of yeast mixed in a couple of tsps water and a dot of unbleached flour and mixed it up in the mixer.  Of course this got it to rise nicely and baked it up.  Fine finished product, though I felt like I cheated.

So I thought, well, let's try again as usual, and the loaf comes out just fine, like all my usual results, save this mystery one.  I wonder what happened?  The starter looks the same, smells and tastes the same.  Was it just on strike? between generations of yeast babies?  didn't like the weird music I was playing in kitchen (traditional choral church music actually, mass settings)?  Maybe I just deceived myself about what it was doing that day.  ?? the mystery of yeast and baking??

My usual mix for a multigrain loaf contains, two or more flours of: barley, spelt, rye, whole wheat, oat, and always a little unbleached for working with it when sticky.  I'm not too obsessive about calculations, and go by stickiness versus weight from experience, which I don't think is the problem given many many repeats of the same process for daily family comsumption over the past 20+ years.

Any ideas?  Thanks!