Insufficient rain (drought), or irrigation
Drought, quite obviously, can be caused by less than normal rainfall,
but it can also be caused by hot or cold temperature extremes where the
top several inches (cm) of soil may freeze, and roots may be unable to
take up water. Reduced water uptake in the winter can make trees, in
particular evergreens, more vulnerable to drying out.
Be sure to water in the winter if the ground is dry, including soil in containers and raised planters.