It comes from violent contact on the ears in the tackle and ruck and -- especially for second rowers who jam their heads between the hooker and prop -- friction on the ears when binding tightly in the scrum. That's why you sometimes see players with a cloth bandage, secured with electrical tape, wrapped around their ears. It's cooler than a scrum cap, but still protects the ears. It's really only a problem for professional players, or very, very serious amateurs.

It's called cauliflour ears You see it sometimes with boxers or UFC fighters too (look at Forrest Griffen)

because they get beaten and rammed into so they sometimes wear hats

What i found on internet: Searching: 'Rugby Player Wierd Ears' Cauliflower ear is a condition that occurs when the external portion of the ear suffers a blow, blood clot or other collection of fluid under the perichondrium. This separates the cartilage from the overlying perichondrium that supplies its nutrients, causing it to die and resulting in the formation of fibrous tissue in the overlying skin. As a result, the outer ear becomes permanently swollen and deformed, resembling a cauliflower