Why? (A computer mystifies me)

So I am trying to get a computer to boot that I haven't turned on for about three months. It refuses to turn on (no bios beeps, no fans, but the motherboard light is on). I replace the clock battery, reset the bios, vacuum the dust out, check that the power supply works, check the power button works with a multimeter, try reseating the power cables, nothing works. Then with the case off, I'm pressing the power button randomly, and one of the times I randomly spin the CPU fan, and then the computer just comes on when I spin the fan. Whiskey Tango Hotel? After that it has turned on a half dozen times fine.