Why are cashiers so snotty acting?

I went to the store today and bought something. i go up to the check out line with my mom. There was a lady behind me. me not thinking, I didn't put my stuff down that i bought on the counter the cashier seen the lady's things and grabbed her stuff and then skipped me. my mom told me that i'm suppose to put my stuff down for the cashier to see, then the cashiers gonna bust out and say "YEAH!" just basically yelled at me and my mom was right there and didnt do anything. Why are cashiers so damn mean?????!!!!