Backfire

I recently bought my 94' Seville. Everything seemed great until today. I went out to start it (already warmed up) and the engine went "poof!" Light smoke rolled out from under the hood. I opened it up and saw the front plastic cover (in front of the injectors?) popped forward on the right side. I pushed it back in place and started the engine without a futher problem. What happened?? Anyone ever experience this?

Just for those who read this, i did come up with a reasonable explanation. More than likely, when the engine was shut off last, an intake was open and allowed fuel to seep in and since it was warmed up already, when I started it, the plugs sparked, igniting the fumes which came back throught the intake. And since it has not happened again, this seems plausable.