I've noticed that lately the Echo doesn't stay off, after awhile the relay audibly clicks back on and power is restored. The time it takes for this to happen seems to be random, sometimes it takes 15 or 20 minutes, but at least one time it was only a minute or two.
My first assumption is that the ESP is crashing and restarting, which naturally returns the relay to its normally closed position. I'll have to pull it off the wall and connect it up to the serial monitor to confirm.
But the bigger question is why? Perhaps the relay is drawing too much current and it's causing something to overload. Or else the USB power supply that's running the ESP/Dot just isn't up to the task and is faltering.