According to rumor, the line "Life does not stop and start at your convenience, you miserable piece of shit" was directed at Joel Coen, when John Goodman thought he said "cut." Source here. However, this reason is obviously a joke propagated by the Coens. The line is in the original shooting script and clearly fits the tone of the scene. Walter is expressing his sentiments about the Big Lebowski thinking he can summon the Dude during league-play. It may even be directed at Donny, since he is (again) attempting to enter the conversation without prior knowledge of what the Dude and Walter are talking about.