I2C Relay Circuit shorting & sparking
Seems like there are probably multiple issues on your board. I can't speak to the sparking you're seeing, but the MCP23017 does require the address pins to be biased, according to page 11 of the datasheet:



I'm not sure if you're trying to use a specific address or not, but your software will need to reference whichever address is selected via these address pins. If you don't care what the address is, I'd just ground all three pins to use address '0100000':