Maybe it was too cold for the thin smaller branches this year so they lacked enough protection? Maybe they kept growing due to too much or late fertilizers so some of the stems got hit with early freezes/frosts? Dry winters? Maybe ... ??? ;o)

Might just be the the lower branches are too shady and they're no longer of use. Might be some short critter chewed on them.

Do they look cut? Could be hungry rabbits as kidhorn2 noted above.

This is rodent damage on this young big leaf hydrangea last year.