Here is my guess, I haven't flown the ATR for a while, so I'll try to recall the best I can.
You could have a drift in the AHRS 1 versus 2. In general case there would be a master caution when AHRS 1 and 2 differ by 6 degrees or more in pitch or roll (pls check the numbers in the book). If it was to occur during a CAT 2 approach, the AP may try to make corrections based on roll/roll rate or pitch/pitch rate, that would not match real life deviations, thefefore the corrections would be inadequate, and an excessive deviation may occur. During a go around you would have to go SBY first, until you can crosscheck both attitude indicators vs SBY, just like you would on AHRS drift during take off.
Flex