Becasue during German propaganda radio broadcasts 'Lord Haw Haw'
as called by Allied troops, would refer to the Tobruk garrison as
"poor desert rats of Tobruk", mocking them and saying that they
were caught like rats in a trap, and that they could not get out.
When he was saying this he was specificaly refering to the
Australian troops, the 9th Division and 18th Brigade. They then
adopted this and became known as the 'Rats of Tobruk'.