The primordial magic of Krynn, before the gods of magic organized it, was described as wild. Then the Greygem was created and wild magic reigned supreme, as the gem changed animals and people and infused magic on living beings, be they dwarves, elves or humans. If you read the Covenant of the Forge (first book of the Dwarven Nations trilogy) you will see that the magic users of the book did not conform to the standard D&D rules.
What if those “infused” with magic by the Greygem were the first sorcerers? Later the gods of magic, with the help of the three elven sorcerers, codified magic and defined it, so High Sorcery was born. The new breed of magic users referred to the previous kind of magic as wild magic or “low sorcery”, spreading legends about its lack of reliability.
The wizards of the newly established orders of High Sorcery hunted down their brethren and forced them to join the new order or die, so were the first “renegades” born.
During the centuries the magic on the blood of the original sorcerers laid dormant on their descendants. Sometimes the power awoke and a new “low” sorcerer walked the land.
During the time of Huma, when gods meddled with human affairs and walked the face of Krynn, the spark of magic that had long laid dormant on the people suddenly awoke and sorcerers were seen in numbers for the first time in years (Galan Dracos may have been a sorcerer, or not). After the gods left, the spark once again went dormant and, because of the lack of godly influence during the Age of Might, almost no sorcerers were seen.
It was after the War of the Lance that sorcerers started resurfacing in numbers. The godly meddling once again awoke the spark and sorcerers once again walked the land.
So there you have a perfect rational explanation (I hope) about why suddenly there are sorcerers on Krynn.