The source explained the issue is caused by a mechanical defect that causes the sub mirror system to become misaligned. This in turn causes the light rays reflected by the sub mirror to miss the AF sensor, rendering it useless. The full technical explanation follows.
This phenomenon is due to the Locking Claw of the Sub Mirror (mirror for AF) going over the Locking Pin. The Sub Mirror’s angle becomes deviated and the light rays for AF does not fall on the AF sensors, causing the ‘does not autofocus’ phenomenon.
While most photographers won’t have to contend with subfreezing temperature, such as -20°C (-4°F), it’s a major issue for anyone shooting landscapes or capturing polar bears in the arctic. Canon has since corrected the issue for cameras manufactured after January 24, 2013. However, if you’re hanging onto an older camera, replacing the mirror box with a newer version labeled CY3-1661-010 or CY3-1687-010 can repair the malfunction.
Via Canon Rumors