bridging happens when too much DE is in the filter and it fills the spaces between the grids. This happens when the filter is backwashed and recharged since it s impossible to know exactly how much DE was removed in the backwash it becomes a hit and miss on how much DE to add. This can and will results in either an undercharged filter which leads to clogged grids (which then need to be soaked and cleaned) or an overcharged filter and DE bridging (which you have seen first hand.
Also, when you think about the water flow during a backwash you are forcing dirty water BACKWARDS through the grids and this can also cause the grids to clog.
I am of the camp that DE filters should not really be backwashed but should be broken down for cleanings for the above reasons.