what are the reasons why a men will have pre-ejaculation on their underware?

Pre-ejaculation seminal fluid, is a lubricating, clear, sticky formula,that cleans the urethra normally before ejaculation.

The fluid is usually secreted (causing a leaking effect) by Cowper's glands during masturbation, foreplay or at an early stage during sex, some time before the man reaches orgasm and semen sperm fluid is ejaculated.

In some men  the leaking pre-ejaculation can happen just by being sexually aroused and excited by looking at porn, or other visual stimulating activity causing the seminal fluid to be leaking on their underware. 

Pre-ejaculation leakage also is a sign of the weakening of the parasympathetic nerve that keeps the ejaculation valve closed and helps to maintain an erection. If this electrical feedback loop develops a problem, eventually erectile dysfunction can develop.