The usual reasons
Iowa State Extension lists these: some species need more warmth before growth starts; cool weather slows bud break; a freeze after a warm spell can kill new growth, forcing the tree to leaf out again from backup buds; severe cold or fast temperature swings can injure buds; spring leaf diseases can kill early leaves; drought the year before can drain energy reserves; and compacted soil, poor drainage or root damage can slow leafing.
It is an out-of-state source, but the causes apply here. A late spring freeze after a warm spell is the kind of event that can set it off.
The wait and the scratch test
The same source says not to judge a tree that has not leafed out until at least June 1. A simple check is to scratch a small twig's bark gently: green tissue beneath the bark means the branch is alive, dry brown tissue suggests dieback.
If a tree has partial leafing, bare branches, failed scratch tests or no bud swelling well after that date, something more serious may be going on, and an arborist should look.
What to avoid, and when to call
Do not prune off bare branches in early spring; a limb that looks dead may leaf out later. Do not apply a fix to a tree you cannot yet judge. Our guide to watering mature trees covers the drought side.
If the tree is large and could hit something, or if you want it inspected after a hard winter, our two ISA Certified Arborists can check the structure. Call 775.232.8243. See drought stress and dieback.
If you want something to do while you wait, water deeply during dry spells and keep mowers and string trimmers away from the trunk. Those simple steps do no harm, and they remove two more possible causes before the arborist visits.