Can someone please tell me how did this happened?

Originally Posted by loiro370z

Maybe this?
Was reading about how to fix bird drooping in my car and found this article.
Copy and paste from the site.

Blistering
Blistering, sometimes called pimples, are bubbles or swelled areas that show up in the paint surface weeks or months after a paint job. They're caused by moisture that's been trapped under the paint surface and is sometimes caused by spraying during really high humidity conditions. It can also be caused by contaminated air lines, failure to drain your compressor, or by painting over an unclean or contaminated surface. Blistering can sometimes be repaired by sanding the affected area and refinishing, but usually one has to actually strip the area to its bare substrate and start from scratch. This malady can be prevented by always draining your compressor and air lines, proper cleaning and prepping before painting, making sure everything is totally dry after wet sanding, and making sure you use the correct thinner/reducer for your spray conditions.

Read more: Help With Troubleshooting Paint Problems- Preventing Blistering And Discoloration Of Paint- Street Rodder Magazine Page 3