If he is rushing because he held off going to the bathroom too long. The minute he unzips his pants he starts peeing before he gets his aim.
Another reason may be if he is an older man with an enlarged prostrate. Most likely his urine will not be in a "uniform solid stream" but instead "spray" or occassionally comes out in multiple blasts.
Imagine using a bottle of Windex which contains various settings for adjusting the spray. When a man pees it's not always going to come out with the same force. However as has been pointed out in other comments, if an accident happens it should be wiped up!