Why does my horse keep getting abscesses

an abscess. Mechanical overload high intake of soluble carbohydrate sugars and starch such as long grass hormonal diseases such as EMS PPID are some of the possible causes of laminitis but the result is a weakened hoof structure leading to further problems within the hoof itself such as chronic frequent abscess formation.
Keratoma formation 
A keratoma is an area of hardened hoof wall that forms between the pedal bone inside the hoof and the hoof wall. This leads to internal separation of the hoof structures which can allow bacterial invasion. Once the keratoma is large enough to disrupt the external hoof wall itself. 
Pedal bone disease e g. pedal osteitis 
Inflammation and or subsequent infection of the pedal bone within the foot due to injury repetitive concussive injury can also result from a bruised sole corns or laminitis. Again disruption to the hoof structure can lead to infection penetration and abscess formation. 
Hormonal disease 
Equine metabolic syndrome EMS is a condition where the horse has an inability to regulate their own insulin levels due to insulin resistance. These horses are often overweight leading to mechanical overload of the structures of the hoof and therefore weakening of the structures making them more prone to abscesses. But also too much insulin within the bloodstream itself has an effect on the growth of the hoof causing it to overgrow and form less strong bonds between the internal structures and the hoof wall. This weakness can lead to cracks laminitis infections and abscesses.   
Pituitary Pars Intermedia dysfunction PPID also known as Cushings
Horses with Cushings disease produce too much natural steroid in their body. This imbalance of hormones can lead to a reduced immune function making them prone to infections such as hoof abscesses but also the excess steroid in their bodies has a direct effect on the hoof structure leading to instability and therefore the opportunity for abscesses to form. Cushings like EMS also leads to insulin resistance and subsequent laminitis making hoof problems a serious issue for these horses. 
Pregnancy
The hormones involved in pregnancy such as relaxin a hormone that helps relax the soft tissues of the reproductive tract in mares about to foal can also act on the structures of the hoof in late pregnancy in turn leading to laminitis disruption of the hoof structure and possible abscess formation. 
So why does it keep happening to MY horse?!
In summary if your horse experiences
ongoing or multiple abscesses you need to consider the environment. Is it safe free of potential penetrating foreign objects clean not boggy? Does your horse have regular hoof care with a farrier? If all of that is up to scratch your vet may need to look for underlying conditions for recurrent hoof disease and infection such as hormonal disease or disease of the deeper structure of the hoof. Investigations may include blood tests and or xrays. Treatment is usually drainage of the abscess followed by treatment of any underlying condition. See your vet if you have concerns about your horse and recurrent abscesses