Sukun transforming into a Kasra?

Bascially, the word قامَتْ is originally ending in a سُكون. However, the next word الصَّلاة is also beginning with a سُكون so this poses a problem when it comes to the word flow and joining between them. You cant join between two سُكون since its simply not possible.

As such, whenever you have this case, the last letter of the word flowing into the next word that is beginning with a سُكون will be transformed into a كسرة. This is to allow us to connect them and facilitate the flow of the sentence.

Khalil Ibn Ilyas Laher