I'm not absolutely certain of the facts, but I rather fancy it's Shakespeare who says that it's always just when a fellow is feeling particularly braced with things in general that Fate sneaks up behind him with the bit of lead piping.

Wodehouse’s speaker mock-solemnly invokes “Shakespeare” to dignify a very un-Shakespearean sentiment: that misfortune tends to strike precisely when one feels most confident. The comedy comes from the mismatch between the high-cultural name-drop and the blunt, almost gangsterish image of “lead piping,” which turns abstract Fate into a petty thug. The line also captures a recurring Wodehouse theme—optimism punctured by sudden reversals—while gently satirizing the habit of attributing any pithy observation to Shakespeare. Beneath the joke is a recognizable psychological truth: moments of self-assurance can invite carelessness, making setbacks feel like ambushes.