The Bais Lechem Yehuda quotes the R. Chaim Vital that the אריז"ל cut his פאות at the point where the beard starts. (mandible connects with skull - mid ear)
The אריז"ל held, פאות longer than this shiur are just simply hair and not פיאות. According to the Zohar*, The פאות (hair) should not rest on or be mixed with the beard. They should be placed behind the ears.
מנהג חב"ד follows the אריז"ל and cut their פאות short for this reason.
There are others who explain the אריז"ל differently and claim that one may have the length of the פיאות up to the bottom
of the chin. (not longer)
Many Chasidim grow their פיאות and never cut them based on the Mechaber's Loshon in Shulchan Aruch לא תגע בו יד.
Even though it's clear from the Bais Yosef on the Tur that he is not referring on the length of the פאות.
*נשא דף קל"א