There are a lot of reasons for this to happen, merchant filling a report, cj randoms affiliate's sites review, but the usual reason is unormal trends in your numbers compare to the other affiliates that are running the same campaign, so watch out for ctr, engaging traffic, earning a lot of money in a short period, not earning money at all, short burst of traffic mix with quiet periods (like a sinewave pattern).
To fly under the radar, your traffic, conversions, and ctr, must grow steadily in a normal random pattern, month after month.