Why do bulls have a hump?
Bison humps are made of muscles, huge muscles. According to this and this pages bisons use their heads for snowplowing. This hump provides head support. It seems that also other animals have structures like that: giraffes, rhinoceroses, elasmotheriums,... this list can go on since this structure should be homologous of other animals'. From this page rhino humps made from muscles, nuchal ligament, fat, and dermis. And from the page, probably fat quantity depends on the nutritional state of animal.

I searched nuchal ligament, the remarkable structure of hump according to rhino page. This is a ligament spans from upperback vertebra to neck vertebra and to skull. Can be found on humans too. Also muscles of hump should be homologous to humans' notice that vertebras have spines not just for supporting ligaments they also for muscles. Human skeletons just don't have vertebra with long big muscle connection points and huge neck/back muscles connecting huge bones, this is why we don't have a hump or an obvious large hump. And I think even camels do have slighly bigger muscular humps but theirs are covered with big fat hump.

Bison skeleton and elongated spines supports huge muscles and ligaments.



Horse's nuchal ligament



Human nuchal ligament



Human nuchal ligament drawn with red lines and muscles (bison hump also consist of muscles).



Giraffes' this is an energy saving feature. Giraffes don't need to use muscles to hold their neck. They just use when flexing their necks down, when drinking water etc.



According to Wikipedia, for an alternative hypothesis Ouranosaurus have a hump. (Other hypothesis is display sail or termoregulation sail of course. Also spinosaurus have this kind of alternative hypotesis but this hypothesis not accepted much as sail. and spinosaurus' spine different from bisons. Bison spines concentrating at shoulder but spinosaurs' not at the shoulder. You can find spinosaurus info from this page.)