It because the road works on the M50 north are descreasing and so traffic isn't tailing back to the firhouse exit on the M50.
When this happens, drivers take on sliproad 'surfing', up one side of the firhouse exit and down the other because they think it would be quicker.
All this means is that traffic heading to the M50 from the tallaght side, can't get onto the roundabout at the motorway exit and this causes the tail back right back to the woodtown roundabout and beyond.
For the last few morning, traffic has being moving well on the M50 north, and hence no reason for the surfing and no tailbacks.
Of course now it has moved to the other side of that junction as the southbound traffic is now being held up by roadworks.