Score bar clearing question

I have been playing for a long time now but have never gotten around to asking this question or being able to figure it out.  I am always seeing my score indicator rapidly reset to blank several times during a playing session without having an extra bird awarded.  My only guess is there is some object in the playing filed that causes the reset but I can figure it out. Has anyone else noticed this or can someone tell me why this happens. I sure could have used some of those extra birds that I missed because of this reset.