Final answer:
During Anaphase A, MTs shorten due to the depolymerization of tubulin subunits at the kinetochores and at the spindle poles, allowing sister chromatids to be pulled towards opposite poles.
Step-by-step explanation:
In Anaphase A, the two reasons that microtubules (MTs) are shortening are due to the activities at the kinetochores and at the spindle poles. First, at the kinetochores, kinetochore MTs attach to sister chromatids and shorten as the chromatids are pulled apart toward opposite poles. This process is driven by the depolymerization of tubulin subunits at the plus ends of the MTs, which occurs at the kinetochore itself. Second, MTs also shorten at their minus ends near the centrosomes or spindle poles, contributing to the force that pulls the chromosomes apart.
While kinetochore MTs shorten to separate the chromatids, non-kinetochore MTs, which are not attached to chromatids, are responsible for pushing the spindle poles apart, assisting in elongating the cell. Hence, the coordinated shortening and lengthening of different types of MTs facilitate the movements required during Anaphase to ensure proper segregation of the genetic material to each of the daughter cells.