It could simply be the temperature. What's the actual temperature in the garage? I would be wary of spraying below 57°F and ideally it would be around 68°F to avoid moisture problems. The evaporating solvent will drop the temperature of the paint significantly on the surface of the guitar (latent heat of evaporation) and you could be getting a small amount of condensation trapped under the paint as a result.

I'd take a thermometer into the garage to check the actual temperature. If possible, try and warm the space up first with a heater, but turn the heater off before spraying. Also keep the parts to be sprayed inside until the last minute and warm the spray cans up in a bowl of warm water.

It could be something more obtuse, like resin in the ash grain lines reacting with the finish, but I'd suggest trying to get warmer spray conditions first.