My Husband has some 100% cotton oxford shirts that have some tiny holes here and there. What is causing these holes to develop?

The Oxford weave consists of two, thin warp yarns to  every soft, thicker yarn in the filling direction.  The unbalanced construction causes the thin yarns to break, leaving tiny holes.  Manufactures could use a high twist in the yarn to retard the development of holes, but eventually any Oxford weave will develop tiny pinholes.  Constant abrasion during normal wear causes only the thin yarns to weaken and tear.