It is ordered in the chronological Order( science of arranging events in their order of occurrence in time).
In a platform like twitter and other social medias when you follow someone or when someone follows you, It sorts the list by the time from actual event.
So that user can know by whom was user followed recently or whom did user follow recently.
Forward chronological order lends itself well to finite amount of data that tells a story. Reverse chronological order lends itself well to (potentially) infinite amount of data that loses usefulness over time.
For example, in Facebook, each post is listed in reverse chronological order, so you scroll back through history, but within each post the comments run in forward chronological order so you can easily follow the conversation.
Check this question (similar)