What caused and ended the time loop in Groundhog Day?

The second draft of the script states that Phil was cursed by his ex-girlfriend. After they trimmed her character from later versions of the script (finally removing her entirely from the film), that plotline made no sense, so they seem to have decided to simply leave it as an un-resolved question:

CUT TO:

INSERT : A THICK BOOK The cover reads "101 Curses, Spells and
Enchantments You Can Do at Home." A well-manicured feminine hand opens
the book to a marked page .

INT. CHERRY STREET INN - NIGHT - SAME TIME : Phil enters his room and
drunkenly tosses his overcoat, scarf and gloves on the floor in a
heap.

INT. STEPHANIE'S BEDROOM - NIGHT : Stephanie Decastro, Phil ' s
disaffected ex-lover, is sitting cross-legged on the floor with the
book of curses open in front of her. Her hair is down, she's wearing a
caftan with a Zodiac print, there are candles everywhere and other
vaguely occult decorating touches.

INSERT : Phil's business card is dropped into a dish. Then the Tarot
card of the Hanged Man, a chicken bone, and a feather are placed on
top of it .

PHIL : He stands at the sink, looking at himself in the mirror, flexing
his muscles.

STEPHANIE : Reading from the book she mutters incantations in a secret
language, then she sprinkles some powder on the plate, then a few
drops of oil. Then she makes a few passes over it with her hands and,
much to her surprise, the contents of the plate spontaneously combust.

PHIL : As he crosses to the bed, he accidentally knocks over the
suitcase stand, spilling his clothes out onto the floor.-25- He
contemplates picking them up for a moment, decides to leave them
there, and flops down on the bed. He lies there looking up at the
ceiling until the room starts to spin around, then he closes his eyes
and quickly drops off to sleep, still fully clothed.

STEPHANIE : To complete the spell, she picks up a broken wristwatch and
drops it into the fire.

INSERT : Phil's business card, the Hanged Man and the broken watch in
flames. The watch crystal is cracked and the hands are frozen at 5:59

It is also worth noting that in the earlier draft, the original spell kept Phil in the loop for 10,000 years(ish) and that it was Rita's kiss that finally broke the curse;

Phil : You want to know what's really amazing? I've been waiting for you every day for ten thousand years. I dream of you every night
of my life. You've been my constant weapon against total despair, and
just knowing you exist has kept me alive. How's that?