Why would my garage door open on its own?

My garage door opened in the middle of the night & it made security alarm go off, If it was a neighbor wouldn't it close back down?! It hasn't happened in over 2 years since we switched frequency. It just opened & it stayed that way. I'm somewhat scared since my Husband is a firefighter & I sleep alone every other night with my 2 small children. I locked it so it doesn't happen again.