I think it is a function of the type of steel that barrels are made of x the corrosive force of heating/cooling.
We have seen this ...and worse on various brands of gas, and charcoal grills. Unless a grill is made of high grade stainless - I usually consider it just a yearly maintenance issue. I chuck up a wire brush in a drill - and knock off any rust - then spray paint with a high temp grill paint such as one from Rustoleum.