Bank platform 9 (DLR boarding)

There are two entrances to the DLR on platform 9 at Bank station. During the afternoon peak, the one at the bottom of the stairs from the Northern line is always closed off. Does anyone know why this is?

It seems crazy to me! It forces everyone through the single remaining entrance which causes that part of the platform to become super-crowded. If we were allowed to use both entrances it would spread the crowd...