Why do "align-items: flex-start" and "flex-wrap: wrap" mess with my elements?

Question

I have a box (div.rack) in my application with icons.

Here's the rack style :

.rack {
  display: flex;
  align-items: flex-start;
  flex-wrap: wrap;
}

(the rack has a fixed height)

This is driving me nuts but why are the wrapped icons not all at the top?