What is causing the input to my buffer to be damaged?
First, you left off the bypass cap on U2.  Second, you have the ground tied together on both sides of the opto-isolator, thereby defeating its purpose.  You might as well connect IN directly to U2 pin 1 via the 220 &Omega; resistor and be done with it.

Separate the grounds and put a 1 k&Omega; resistor in series with U2 pin 1.  If the isolated circuit on the left has high dV/dt, then the capacitive coupling could cause momentary high currents into U2.  The 1 k&Omega; resistor will help limit that to hopefully survivable levels.

Another possibility is that you're zapping the parts in handling.  Make sure to observe the usual ESD precautions, like non-static work surface, grounded wrist strap, proper packaging, etc.