What Causes Frequent Muscle Tears After Exercising?

I work out 5 days a week cardio everyday and add strength 3 times per week. I also walk at a brisk rate doing flat, hills, and steps 4 days a week for about 40 minutes. Here lately about everyother week or so I end up stressing either the muscle in the back side of my left leg or the calf muscle. sometimes it corresponds with a new work out but sometimes not. I typically warm up for 5 minutes at the beginning and cool down for about 10 minutes after with mostly stretching what am I doing wrong?