Few lines of poetry manage to be this small and still say so much. The speaker describes a slip from the "straight and narrow," the phrase we use for living correctly and staying on course, and ends up somewhere unexpected: a roundabout. The joke is in the image. A roundabout is a road that keeps you moving without taking you anywhere in particular, so the speaker is busy but going in circles.
What stands out is the tone. There's no panic or guilt here, and no dramatic confession. The last line, "Not exactly sure how it happened," sounds like a shrug, a little embarrassed and a little amused. The speaker seems unable, or unwilling, to explain the mistake, which makes it feel familiar. Most of us have drifted off a plan without noticing the moment it changed.
The mood is wry and light, but there's a thread of unease underneath. Being stuck in a loop, without knowing how you got there, is funny until you think about how to get out.