One monitor off of the dock, and one direct attached? Maybe this is due to slight variance in startup time (wake from sleep) of the monitors. And also, maybe M

One monitor off of the dock, and one direct attached? Maybe this is due to slight variance in startup time (wake from sleep) of the monitors. And also, maybe Mac is sensitive to that order. Try plugging one in, wait for it to display desktop, then plug the other. I bet you get 100% consistency then.