This type of failure occurs when the retainer is hitting the rocker or the locks are incorrect or lock location is high and possible even with the top of the stem and the rocker or the roller tip is hitting the lock.Anything that creats a movement between the lock and retainer will cause this type of failure.Also look for coil bind and retainer to guide clearences.Look for witness marks at the locks and retainer for movement and at the underside of the retainer and top of seal and guide.Don't rule out valve spring float either.