I just bought a new bike Marin Pine mountain-1 which has 1 X 11 drivetrain. I want to know is Back peddling normal ? I called my mechanic and he said you should not do back peddling or this will happen. when I do that it suddenly starts shifting the chain to lower cogs. I am not sure why ? chain and everything is just 1 week old.
This is typical-ish behavior, especially if your derailleur gets a bit out of adjustment/alignment, the chainline not being quite right, etc.
You shouldn't really be backpedaling significantly, just to move the pedals into the right place (e.g. cornering, starting to ride, etc.). So, this isn't really a problem in real riding. If its happening when you're just moving the pedals (which is about a half turn of the crankset, normally, like getting the pedal up to start riding), then take it back to the shop and get them to check the adjustment of the derailleur.