There are twp reasons this can occur.  Typically this is related to #2, where the frame rate is higher then what the chipset can provide.  Some applications do not correctly set the frame rate, which will cause an overflow to occur.  

1) When there's a change of size of GOP (Group of Pictures) in MPEG AND there are still some readers left OR there's a format change and there is still something in the video queue. 

2) It's returned to the reader when there are no other readers and our bandwidth is depleted. The step of bandwidth is calculated as max(fps/interval,1) if it's coded in D1/HD1 and max(fps/interval,1)*2 if it's CIF (default), and the max bandwidth per driver is calculated as fps*20 or fps*16 (in case of solo6010). It is done not to flood the system.