Why did Anderson shelters have grass on them?

Answer 1

Anderson shelters were typically semi-underground and covered with a thick layer of earth and grass to act as extra protection and insulation against damage from flying debris, and also to protect them against the overpressure caused by high explosive blasts.

In an explosion, air is sucked inwards towards the epicentre of the blast, which could have meant that an uncovered, unprotected shelter full of oxygen would implode, having it's door torn off and the air inside it drawn out with such violence that the shelter would be crushed like a tin can, killing or injuring it's occupants in the process. This is why it was necessary to situate them semi-subterraneanly, beneath a thick earthen layer to prevent this from happening and with the doors facing AWAY from the most likely area of attack.