This property must be due to a conformational shift in the tubulin that causes it to lose some affinity for its binding partner in polymerization. This is an excellent mechanism to utilize for dynamic instability because it is not too complex and can occur quickly within the cell. In addition, the cap is an important part of this process because it prevents the microtubule to completely degrade.
One of the most fascinating qualities of Tau protein is its incredible stability. Even in its regular form, Tau protein can withstand the heat of boiling and not denature. When the protein begins forming aggregates, it becomes even more difficult to denature and researchers believe that this property contributes to the mechanism by which tau build-up kills other cells in its vicinity. It takes up excess space and essentially suffocates the cell.
While I realize that microtubules are involved in the process of motility in cells, I always thought that actin filaments were the primary driver. As described here, the microtubule push outward onto the cell membrane and cause the cell to move. Perhaps microtubules are more involved in the directionality of the movement, rather than specifically generating the force necessary for the cell to move. That would leave actin as the protein with the purpose of generating the force required.
This concept is one of the most interesting jobs of microtubules in my opinion. Especially because no one fully understands how they influence gene regulation. I've heard that microtubules in the cytoskeleton form in a specific way once the cell differentiates and results in certain regions of the DNA to be unreadable. Therefore, each cell type will have a specific set of protein that it is able to transcribe and translate. The DNA that a given cell has access to after differentiation is dictated by the microtubule cytoskeleton and accounts for the proteins that give each cell type its specific identity.