Alisha pees in her pants on purpose in order to avoid possibly having an accident later in this video.
In this scene we find Alisha at the end of a video shoot for HD Wetting. All the planned scenes have been shot, and she is about to get in her car and drive away. However, she is facing more than a two hour drive to get home and has been drinking lots of fluids for all the scenes she has been filming. She is concerned she is going to get on the road only to find herself desperate to pee.
So, as a preventive measure, she chooses to pee in her pants now, before she starts driving home. This way, she figures, she is starting the drive with an empty bladder and will hopefully be able to avoid having an accident on her journey.