The root cause is increased wealth inequality. "Capital" is assets that are invested by their owner instead of just immediately consumed. So, the amount of capital that exists is variable, depending on the preference of asset owners, which is a function of the rates of return they can expect, as well as the utility of consumption. So its backwards to to say "rates of return go down because there's too much capital" - what's really happening is that a lot of assets are held by people who are so rich that the marginal utility of additional consumption to them is essentially zero, which in turn means that they are willing to invest their assets until the rate of return is also close to zero.