I read today about the HW infobox and figured out how they act like tank for units behind them.
T West made two videos about this, where he explains why villagers can safely repair rams or mangonel and cannot be shot by towers or castle arrows.
I wish I could explain with my words but english is not my first language. 
For the sake of completness I did some further tests since I did not know whether the machanic works also for units behind enemy rams:
castle (player 1) - ram (player 2) - villager (player 2) : castle cannot shoot villager
castle (player 1) - ram (player 1) - villager (player 2) : castle can shoot villager
castle (player 1) - ram (player 2) - villager (player 3) : castle can shoot villager (player 1 and 2 allied)
castle (player 1) - ram (player 2) - villager (player 3) : castle cannot shoot villager (player 1 and 2 neutral)
castle (player 1) - ram (player 2) - villager (player 3) : castle cannot shoot villager (player 1 and 2 enemies)
Not really sure, but I think we should wait for a T-West or SotL video to come out to fully understand how this “new” mechanic works
The castle and fire ship hit the first unit in their path and hence are unable to hit any units behind them. The Hussite wagon however works differently. It doesn’t absorb the projectile. Instead the projectile deals 50% less damage to its target.
The Hussite wagon itself takes 1 damage when this happens. Projectiles with blast damage like mangonel/bombard cannon/scorpion do full damage. The Hussite wagon still takes 1 damage however.
It affects trebs also. This means if you place a Hussite wagon in front of your trebs, they will only take 25 damage from enemy trebs while your trebs deal the full 50 damage to them. It also protects your castle from enemy trebs. Castles take only half damage if there is a hussite wagon in front of it.
Now i think that the wagon cover for units behind means its hitbox is larger. It’s effect decreases 3 tiles away and longbowman looks more effective than xbox to shoot units behind HW. (The way the arrow “flies to the target”, i mean)
assuming this is aa glitch, similar to the cousti doing 2x dmg v archers on release…