I've finally figured it out!  I always thought it was funny that they back
up when they do their "thing".  Friends always comment about it and I say
"that's just the way they do it".  But there really is a reason.  Very smart
when you think about it.
 
We've all heard the saying "keep your back to the wall", well that's what
they do to protect against an ambush when in a vulnerable position.  The
beep, beep is the signal we use as a warning of a potential "illegal
dumping".  :-)
 
SP
[Posted in FML issue 2188]