Let me give it a shot, in hope to be corrected if I have wrongly understood the concept.
A good number of bears want to square off their positions as the prices are not going their way. As the market nears closing, they get desperate and start hogging away higher offers. They are in no position to make a bid. They are 'squeezed'.
Of course! bulls are also buying. This coupling fuels the upmove, making it severe.
And we call it the "Dance of the bears...."
<Or was it Belly Dance>