Mine did this too -- not very often, but random. Minimized it by setting the case fans to 12sec delay on both spin up and spin down and tweaking the mid-range temp, but they still did it. Also had a longer delay to bring the CPU fan up to full speed compared to what I set; had to move the high temp trigger point down a little to make it work consistently.
Then I switched to not using AI Suite at all, and tweaked the fans all in BIOS, and the problem is gone. Only occasional slight increases in speed that directly correlate to background CPU activity.
HINT: Use the Q-Fan tuner curve to drag your trigger points for case and CPU fans -- it's available off the top of the BIOS screen. For the CPU and case fans, set the delays in the Q-Fan control area on the Monitor tab, but
not the trigger points. The temperature/speed setpoint values have some arbitrary limits here, but if you leave these alone the BIOS will use the values you set with the click-and-drag control in the
separate Q-Fan screen. Just set your spin-up/spin-down values here. If you're controlling your water pump with the BIOS, you have to use the settings for it in the Monitor tab. If your pump allows variable speeds, you can set your range there.
Motherboard: Maximus VIII Hero
Processor: I7 6700K - 4.9GHz OC
CPU Cooler: Corsair H80i v2
Memory: G.Skill DDR4-3000 16GB
Graphics: EVGA GTX 1080 FTW2 1721/1860MHz
Storage #1: 1TB Western Digital Black
Storage #2: 2TB Western Digital Black
Case: Antec Nine Hundred
Power Supply: Antec Edge 650 80+ Gold
OS: Windows 10 Pro