Where do the terms "Law of Wheels/Cycles" come from?
By interview of Gen Urobuchi, Law of Wheels/Cycles is a folklore talked between Magical Girls.

Following idea is my (and some Japanese anime fan's) guess.

Law of Wheels/Cycles is translated from "円環の理".
理 is principle or reasonable.
I think 円環 is circle or ring instead of Wheels/Cycle.

Madoka is officially written as "まどか", but most Japanese have a name in kanji.
By a naming dictionary,
most common kanji for the name "まどか" is "円"(circle).

By this two facts, I guess 円環 means Madoka.
In the rewritten world, most people forgot about Madoka, but Homura found some affected things from Madoka.
I guess the folklore was named Law of Wheels/Cycle because of such a effect.