Population Migration / Travel
Portals between the Feywild and the Material Plane began to spontaneously appear, especially near the lairs of dragons. Fey creatures began to cross over into the Material Plane, initially only a few curious and adventurous individuals.
From its very inception, the Feywild was a place of ambient arcane power. When Keryth's Sundering loosed the dragons as sources of arcane magic in the Material Plane, the power emanating from them gradually weakened the barrier between the two dimensions, causing the spontaneous formation of Fey portals connecting the two.