At the very end of the episode 5 of season 6 of The Walking Dead, a growing blood trail is shown going from the middle of a wall section. It doesn't look like someone died up above because there are other horizontal bars above it that would block the flow. What is going on?
The obvious implication is that the herd outside the wall is now so enormous that the zombies closest to the wall are being crushed by the sheer weight of the horde behind them.
In the first episode of the season, we see something quite similar happen when the herd is being led around the bend in the road, at the point where a temporary wall was erected to direct the zombies in the desired direction. The mass of the herd squashes the zombies who are unlucky enough to be between their colleagues and the wall, and when the herd passes, the walls are smeared with gore.
The same idea applies to Alexandria's walls.