In this dialogue:
A: 昨天他开车的时候,头撞了一下。
B: 怎么一回事?
Why is the classifier 回 used instead of 件?
There are only a few phrases made up from 回事:
怎么(一)回事: what the matter with ..., why ..., what is the reason for ..., what happened
这回事: The matter you just mentioned (E.g 没这回事。 There was no such thing)
一回事: The same. (E.g. A和B是一回事。 A and B are the same (matter).)
In the above situations, "件" cannot be used.
Normally, a "piece" of business, 事, is represented by 一件事. This could represent a contract, deal, or whatever.
But in this case, the "business" is a car accident, i.e. an action. 件 doesn't work as a modifier in this instance. You need an "action" modifier like 回, which roughly means "to come and go."