My bladder leaks a little when I do not have to pee, what causes this?

Pressure. In females the urethra is only a little over an inch long so the leaking is due to the angle of the urethra to the sphincter of the bladder as the bladder drops with age and the inability to empty the bladder so pressure builds up. In males the pressure builds up between the bladder and the prostate so if the prostate is swollen it takes time for the pressure to resolve and leaking occurs.
Depends. Since I do not know whether you are a male or female. For older men: cause mostly benign prostatic hypertrophy with or without cancer. For women cystocele and weakness of the muscles. These can be corrected by surgery with good results. If there is a prostatic or bladder mass, then biopsy under ultrasound or ct scan guidance is indicated to rule out cancer, urologist should be consulted immediatel.