Diagnosis
The fender is mounted with two plastic brackets that fatigue and crack. The fender itself also flexes.
How to Fix It
Inspect the two white plastic mounting brackets attached to the fork. Look for cracks. If intact, remove the fender by unscrewing the two bolts. Place a small piece of rubber bicycle inner tube or foam tape between the fender and each bracket before re-tightening.
If That Didn't Work
The brackets are likely cracked. They must be replaced. Order part number INK-FENDER-BRACKET-LT. The replacement is a direct bolt-on.
Heads Up
Overtightening the fender bolts will crack the new brackets. Tighten only until snug, then a quarter-turn more.