Initial reports were it was a $100,000 watch, but seems like that's been edited from the story:
ABC7 learned a mob of people surrounded Buehler and his wife . . . . That's when someone allegedly unbuttoned his shirt sleeve and took the watch.
Here's the LATimes quoting his management company:
“While walking from the paddock to their seats, Walker was unknowingly the victim of a snatch and grab robbery of his watch. The incident was reported to police and is currently under investigation.
Also in the LATimes's write-up:
According to Arcadia police, officers responded at approximately 5:30 p.m. to investigate three separate reports of grand theft. Two of the robberies occurred while the victims were surrounded by a large crowd of people. The third incident involved three people approaching the victim, with one suspect attempting to embrace the victim while another removed the watch.
...
“Based on the initial investigation, it appears all three victims were targeted by different, organized groups of people
I didn't see an article address whether such crimes are commonplace at the racetrack, or if there was something unusual about that day.