Drought could do that and worse but, you would be best qualified to answer that. Has the weather been dry? Is the sprinkler or drip irrigation working correctly? If you insert a finger into the soil to a depth of 4", the soil should feel moist. If it feels dry, you need to water with about 1 gallon of water. About 3-4" of mulch can reduce the frequency of waterings.
If unsure when to water, test the soil in the mornings daily for about two weeks to determine if the soil is dry, moist or wet. If it feels dry, water and make a note on a wall calendar saying that you watered on that day. If moist, do not water. If wet, try to determine why is it wet as too much water can cause root rot. After two weeks, review the information on the calendar and dettermine about how often you had to water. Then set your sprinkler to water 1 gallon of water on that same frequency (every 2/3/4/etc days for example). If the temperatures fall/rise about 10-15 degrees (and stay there) during the day, re-test with the finger method. Once the plant goes dormant, you can simply water every other week if the winter is dry. Restart testing when the plant leafs out.
Disease can also cause these problems but you should also notice some symptoms or problems in the leaves/stems. Voles and other pests can also cause similar problems if they damage the roots.