If he's angry that everyone was getting into the two guy's biz, why would he do it himself?
Because he's not getting in their business, his attack was at everyone else in the yard whether they be super powered interlopers (as William sees them) like Julian and the other NPCs or the gawkers. If the brothers use the confusion to leave and settle their problems away from prying eyes that would probably be ideal for William.
The towels will move in a straight line without stopping unless they're destroyed or are outside the range I stated earlier. They will push against anything or anyone in their path without stopping as well, so people will get pushed until they're 30 feet away from William or they'll be knocked off balance and probably fall. There are 4 ways to deal with the towels: get out of range, dodge them, destroy them, or have a power which can directly counter the physics behind it.
The dish soap wouldn't have the force of a water hose, a person would just be slowly shoved back 30 feet if they didn't dodge a squirt from it. His power doesn't amplify force it simply causes objects to be unaffected by anything that would change their velocity (air friction/gravity/obstructions/etc).
To explain his earlier fears about his power imagine if he were to accidentally bump a table with his power active, he could easily crush someone against a wall or even push them through it.