[opmelogy]

> why did (apparently) some large proportion of all valley tech businesses park their working capital at the same bank? Is that something we'd expect to happen in a free market?

What's the concern/issue your brain is picking at with this? Based on what I'm getting from your comment you are asking why large groups of people clump together and end up following what's popular/well known. To me, that me says more about psychology and less about the idea of the free market balancing things out. (Not disagreeing with you and not aiming to be confrontational - sincerely curious.)