Here is a few Causes of Prostatitis 

Risk factors include bladder outlet obstruction (e.g., stone, tumor, BPH), diabetes mellitus, a suppressed immune system, and urethral catheterization (i.e., a small tube inserted into the bladder through the urethra to drain urine). Some sexually transmitted diseases (STDs; e.g., non-gonococcal urethritis, gonorrhea) increase the risk for developing bacterial prostatitis. Unprotected anal and vaginal intercourse can allow bacteria to enter the urethra and travel to the prostate.

Acute Bacterial Prostatitis

Bacterial prostatitis is caused by the growth of bacteria that are normally found in prostatic fluid, such as Escherichia coli and Klebsiella. Approximately 80% of organisms causing ABP are aerobic gram-negative organisms (e.g., Escherichia coli, Enterobacter, Serratia, Pseudomonas, Enterococcus, and Proteus species). Urine that flows back into the urethra (urine reflux) that enters the prostate can also cause the condition.

Chronic Bacterial Prostatitis:

Sometimes bacteria remain in the prostate following acute prostatitis. Catheter tubes used to drain the urinary bladder, trauma to the urinary system or infections in other parts of the body can sometimes be the source of the bacteria.

Chronic Nonbacterial Prostatitis:

01 Heavy lifting: Lifting heavy objects when the bladder is full may cause urine to back up into the prostate.

02 Certain occupations: Occupations that subject the prostate to strong vibrations, such as driving a truck or operating heavy machinery, may play a role. Though biking is a good exercise for the rest of the body, it may irritate the prostate gland.

03 Structural abnormalities of the urinary tract: Narrowing (stricture) of the urethra may elevate pressure during urination and cause symptoms. Sometimes prostatitis is caused by a sexually transmitted organism, such as Chlamydia. However, most cases are caused by infections that are not sexually transmitted. These infections can't be passed on to sexual partners.