The most likely causes are:
Symptoms of Leaf scorch, caused by the fungus Diplocarpon earlianum, include reddening and purpling of leaves. These symptoms result from growth and coalescence of smaller purple lesions.
Petioles, runners, fruit stems, and sepals may also be infected. Varieties differ in their susceptibility to this disease. More strawberry leaf scorch information
Nutrient deficiencies such as nitrogen or phosphorus cause purplish to reddish-colored leaves. Phosphorus deficiency is most common when the pH is too low (<5.5) or too high (>7.0). More strawberry nutrient deficiency information