ETA: It's not so much Elly over-reacting to a non-event as it is a character living in fear of her having a melt-down over something harmless while simultaneously blaming other people for his own actions. It's not as if he has free will or anything; if he did, he might have to accept responsibility for his mistakes and that would be mean, unfair and wrong.
Panel 1: We start off with Mike and Lawrence looking at water gushing out of the downspout of a house. This is a common phenomenon: children transfixed by the melting of snow off a roof during a warm period.
Panel 2: Lawrence points out that that's a lot of water.
Panel 3: Out of nowhere, he asks Mike, who was up until that point playing stature, if he's really gonna stick his hand under there.
Panel 4: As he does, Lawrence suggests that he now put the other hand under the melt-off.
Panel 5: Next, he tells him to take a drink of water laced with roof tar, bird crap and the issue of coal-burning chimneys.
Panel 6: Next, he should put his head under.
Panel 7: At this point, seeing that his friend has no resistance to peer pressure at all is so funny, Lawrence is bent double laughing as he tells Mike to go all the way under.
Panel 8: As Mike stands there, soaked to the bon, Lawrence exclaims "Holy cow, Mike!"
Panel 9: The realization that he was being laughed at and not laughed with snaps Mike out of his reverie; it also makes him tell us that he's soaked and Elly will get in his face because of it.
Panel 10: As he yells that it's all Lawrence's fault, his friend is struck dumb. This is because he remembers that Mike could have said no at any time but didn't.
Summary: Pattersons aren't big on accepting responsibility for their own failings. Just as April would refuse to admit that she'd treated Becky shamefully or how Elly would never admit that Kortney took advantage of her, Mike refuses to admit that the people who take advantage of his need to please are right to mock him.