Most likely your enlarged prostate pressing on your uretha, or your
enlarged prostate's median lobe protruding into your bladder and
acting as a stopper as it drains.  Some men can have a very enlarged
prostate with little or no urinary symptoms because the prostate is
not providing pressure on the uretha or protruding into the bladder.
Other men can have small enlargements which provide a lot of pressure
on the uretha and therefore lots of trouble urinating.