The most common cause for joints to open up is:

The most common cause for joints in Hot Mix Asphalt (HMA) to open up is indeed insufficient material at the joint. When there is not enough asphalt material applied or compacted in the joint area, it can lead to reduced stability and inadequate adhesion between the adjacent layers. This deficiency often results in the formation of gaps or cracks, making the joints susceptible to environmental factors such as moisture infiltration and temperature fluctuations. Over time, these joints may worsen, leading to further distress in the pavement structure.

In the context of asphalt pavement construction, it is crucial to ensure that joints are properly constructed with adequate material to maintain the integrity of the surface and prevent future issues. Well-compacted joints are essential to create a strong bond and effective load transfer between the segments of asphalt.

The other options touch on various joint configurations and conditions, but they do not address the primary issue of having too little material at the joint, which directly contributes to the problem of opening joints.