A 'baaa' is when someone copies you so obviously, that you can't help but scream on top of your lungs 'baaa'. It is referencing the sounds that a sheep would make, suggesting the person you are targetting is copying you just like one.
In more friendly, informal environments, the abbreviation is often followed by an actual vocalization of the phrase "balls and wieners" after a few repetitions of the abbreviation by different speakers.