Here are a couple of guesses... Of course I have not sprayed acrylic enamel in 15 years or so.. Spraying paint in direct sunlight or on a surface that is too warm can cause acrylic enamel to blister. Like cold paint on a hot surface. Spraying on a damp surface can cause it too. Excess humidity.... I have heard of folks using lacquer thinner on their tack rags and that can cause it, usually that is a streak.. The paint will fry of sorts.
If the first coat went down smooth and then blistered on the second coat you may not have let the first coat flash off enough. As the thinner evaporated it caused the bubbles... maybe...
I don't know if that helps or not.. I guess it is just an opinions.
