Well drillers will often use a product like Benseal to seal the casing borehole. Sometimes during heavy draws and/or heavy rains, the Benseal may leak into the well. The more Benseal that leaks in, the greater the leak becomes. Of course it could also be naturally occurring clay.
You don't say what size the casing is so we cannot calculate how much storage the casing has. The more the storage is drawn down, the greater the hydrostatic pressure between the groundwater and the well. A 2 GPM recovery is nothing to write home about, so not drawing down the storage too far should help reduce the leakage.