It’s generally because we are travelling less than 2 seconds from the vehicle in front of us.
Keep in mind that we will need to claim on our insurance (and pay excess $) if we run into the rear of another vehicle and the possibility of a fine and recorded traffic offence for Careless Driving.
The easiest way to check your distance is to pick a reflector on the road (in line with the white line and at continuous intervals), or a white stop line, tree, sign post etc and when the car in front passes that spot with its back wheel, start counting until reaching the same spot.
If you count “One Mississippi, Two Mississippi, Three Mississippi”, it takes a second to say it rather than “One, Two, Three” which obviously wouldn’t be Three Seconds.
If you don’t reach the completion of Two Mississippi, then you are too close to the vehicle in front of you, so you would need to slow down until within that safe distance.
Keep in mind, the recommended distance in the dry is 2-3 Seconds and in the wet it’s 3-4 Seconds.
VicRoads ‘Road to Solo Handbook’ (link at bottom of our homepage).