The grenade bouncing on barrels was not a completely intentional feature, but more of a side effect caused by the calculation of grenade behavior. We found the behavior to be pretty interesting, so we decided to keep it in the game instead of patching it up :)

For those interested: Aiming a grenade from a certain distance onto a barrel, might make it bounce off in weird angles and it's possible to shoot over obstacles this way. Not sure if this is intended, but it makes for some surprising mortar action! :D