Fusel alcohols are higher-order alcohols (chemically more complex than ethanol) produced during fermentation, contributing a hot, solventy, sometimes harsh character when present in excess — genuinely different from the pleasant warming sensation a well-balanced strong beer carries, and a real flaw rather than a stylistic feature at any style's strength.
Fermentation stress is almost always the actual cause
Fusel production rises under conditions that stress yeast — fermentation temperature running too high, gravity high enough to push yeast past its comfortable working range, and underpitching (too few healthy cells for the batch size and gravity) are the three most common causes, and they compound each other; a high-gravity beer fermented warm with an inadequate pitch is stacking every risk factor at once.
Why every strong-beer style page on this site repeats the same warning
Doppelbock, Russian Imperial Stout, Belgian Dark Strong Ale, and Barleywine all carry the same core advice for exactly this reason — a healthy, appropriately-sized yeast starter and good aeration at pitching aren't optional refinements at high gravity, they're the primary defence against fusel character that no amount of conditioning time fully removes once it's present at real intensity, though moderate levels do soften somewhat with extended aging, which is part of why big beers benefit from real cellar time before final judgment.