Police say the security guard walked out of a Ross store after the argument on Saturday afternoon, but later returned with a gun and fired at the female manager.
"The man went out very fast, and maybe in two minutes he was back, and firing the gun," a shopper said.
Customers ran for cover and some laid underneath clothing racks to hide.
When police arrived, the guard opened fire at the officers.
He was eventually shot and transported to the hospital in an unknown condition.
The customers or manager were not hurt during the shooting.