Were clothes left in the dryer or washer?
Remove the load as soon as the dryer cycle ends. If the load sits for too long after the end of the cycle, wrinkles can set into the fabrics.
It is also important to promptly remove loads from the washer to avoid the same problem.
Was a large load or mixed fabric load being washed?
Overloading the dryer can cause wrinkling, too. A crowded dryer keeps load items from tumbling freely, which slows drying and increases wrinkling.
Mixing fabric types also increases wrinkling. Lightweight synthetics and blends dry faster and can overdry and become wrinkled, while the 100% cotton items continue to dry. Dry similar fabric types together to reduce wrinkling.
Check clothing fabric care labels for recommended drying temperatures. Temperatures too high for the fabric may cause overdrying, which will result in wrinkling.