Semen production goes on in the testis for 24 hours and gets stored in seminal vesicles. Now imagine seminal vesicles as a water tank. 24 hours production at some time, will make tank full. Its storage capacity runs out. Now overflowing mechanisms take place like night falls and seminal discharge in urine. To avoid this u need to have sex or masturbate regularly. So it's normal, no need to worry as it is overflowing mechanism only.