Lets say your motor, with + 5V applied, runs clockwise at 10000 RPM. If you disconnect it, put a light load,say, 1000 Ohms across it's leads and spin it with another motor at 10000 RPM clockwise, it would generate close to -5V across the load resistor. So, when you put + 5V on it's leads and it's running, it's also generating close to -5V, bucking the +5V you are putting in (that's called back EMF). That's why it's only drawing the low current. Now, if you put a heavier load and slow it down, the back EMF will be less, so the difference between applied voltage and back EMF will be greater, causing current to increase. More current means more torque, so the speed stabilizes at a lower value.