Do you mean it does this constantly, or just once? If just once, then I'd say that's when your thermostat is opening, at the temperature that matches 3/4 on your gauge. If it happens constantly, then you have to fish for other problems. Some possibilities are a) the cooling system was not properly bled of air and b) your cooling fans are not going on and off at the right time and c) your temperature sensor is putting out an incorrect value for the gauge.
It might be a good idea to check coolant temperature with a scan tool to see if it's really what the gauge says it is.