I am not a rescue so I cannot answer for sure, but I have posted many of their stories here on EBN, so the reasons owners give them up vary.
Sometimes, it is because of a move, a divorce (which I don't understand how neither would want to keep their bully baby!) or they simply get "lost".
But I have seen all too many that were used for breeding (especially females) and then thrown in the streets and picked up as a stray. Can you believe they just dump them out like garabage when they are done? Disgusting!
Then there is dog fighting. Like @Champ. Used for dog fights and then thrown into the streets.
Then there is probably the bulk of them that are simply given to rescue because the owners are just tired and don't know what to do to help their bully, vet bills, expenses, smell, skin infections, allergies.... one of the reasons we wanted to create this site. To help bully owners with information on what they can do to keep their bullies happy and healthy, so they don't end up in rescue. I know we have already helped so many, and it brings tears to my eyes that at least those bullies did not have to go to rescue.
But I think maybe if we ask this question to a couple of our rescue friends, they can give more insight about how and why so many are in rescue. I am a third party looking in- so of course I tend to look more toward the drama and sadness because it tugs at my heartstrings. But those who do this everyday will definitely have the answers you are looking for.