Kaiser bun should be spelled with a either capital or lower case. Either is acceptable, so I guess you don’t need a why. Here is what the OED says: “Kaiser (also kaiser) roll N. Amer. [tr. G. Kaisersemmel], a round roll made from unsweetened dough with several edges pulled and folded in at the centre to form a radial pattern.” So you see it is either. But I suspect the reason for the capital, when it was the only accepted version, was because it was named after the original Roman Emperor or the later or German Kaiser (King or Emperor, roughly). It is the same, obviously, as “Caeser” which is generally capitalized when used in that way. Such as “Caeser Salad,” right?