The strength of the wake vortices is proportional the the aircraft's weight so heavier aircraft produce stronger weights. The 757's max weight fell below the "Heavy" category, but higher than other non-heavy aircraft. It was initially treated like any other Large aircraft, not a heavy, so tighter spacing following a 757 than following a Heavy. It had pretty strong wake as compared to other aircraft in the Large classification.

Another problem is the 757s excellent climb performance. It was able to climb more steeply than most of the aircraft hat might be following it which put those aircraft into its wake more often than would normally occur.