Why did my switch spark when I installed it in a metal junction box?
Don't downgrade to a plastic box.  Metal boxes are better and what's more, they are often essential in distributing grounding to other boxes.  Further in some cities (e.g. Chicago) they are required by Code.

The purpose of a box is to provide grounding and fire protection, and metal boxes do that better than plastic ones in all respects.  Plastic boxes exist to be cheap, so builders can save $20 per house. 

Metal boxes also allow there to be fewer (or no!) ground wires in the box.  For instance this smart switch doesn't need a ground wire since it can ground via the yoke and screws. 

This is a quality product with an ETL listing (equivalent to UL listed). Bad designs just won't clear that listing process, so I don't buy "all 4 screws bottomed out on the box".  Far more likely: 


A wayward bare ground wire in the box touched one of the screws (feels unlikely) 
Wires were over-stripped, leaving bare length sticking out.  This contacted a ground wire. 


The screw issue could be annulled with a couple layers of electrical tape across them.  That's how you solve that problem (if it even is a problem).