i will tell you how a stock is shorted by accident...it happened to me.
i opened a new td ameritrade apex account because i was still using an older ameritrade account with different features. i placed a sell order the day that tdameritrade moved my funds to my new account. the sell order went through however tdameritrade screwed up and ended up shorting that stock. do not ask me how they did it but i had to call them to have it corrected.