Catch-22 is notorious for not following a chronological timeline, but instead jumping around haphazardly in time. In fact, the novel doesn't jump around at all; it is in perfectly chronological order as Yossarian perceives it. You see, Yossarian, like everyone else in the novel, is crazy. Crazy people perceive the world differently from normal people. So, when Yossarian wakes up in the hospital after being attacked by Nately's whore, what he perceived as the most recent event was actually the one that loomed largest in his memory (the incident with Snowden). Meanwhile, meeting the Chaplain was perceived as the earliest event because the Chaplain is Yossarian's oldest friend in his mind.