One aspect of Billy's character is the fact that he has incidents of "time shifting" recurrently. For example, he could see his whole life from beginning to end in one "sweep".
In the chapter when Billy is transported to a camp in Germany, he and the other prisoners are taken to a feast by some English officers who were captured earlier in the war. Billy gets a shot of morphine that sends him time-tripping again. His mind was very likely to feel this sensations when some sudden circumstances appear.
