2 blown ignition coils, 5K miles later I had to replace the other 3.

In July, my check engine light went on, and the car started running very sluggish. I had just recently added 2 quarts of oil. I was down a couple quarts. The mechanic replaced an engine coil and just before I picked it up, he said another blew and replaced it too. Now 5K miles later, added 2 quarts of oil again and the next morning the check engine light went on, the car started running very sluggish, brought in for repair, and the mechanic said I needed to replace the remaining 3 coils. He couldn't offer an explanation for the cause. He's a NapaCertified, AAA and all that.
I would like to learn more about the cause and prevention of this problem.