Corners of shingles turning up is referred to as Curling and it is generally caused by excess moisture attacking the underside of the shingle. Inadequate ventilation causes the entrapment of heat and moisture in the attic. This will eventually penetrate the roof deck and cause the shingles to curl.
Improving air circulation within the attic space can stop the progress and rectify the situation if the curling is not too severe. In some rare instances, curling can reveal itself only under cold weather conditions. This would be referred to as winter curling. The corners of the shingles will slightly curl up from the roof deck when cold, then lay flat again during warmer weather.