Commodity trading in India stops at 12 am and is open on weekdays only(off on Saturday and Sunday worldwide) but other countries in the west, keep trading due to the time gap on weekdays. So sometimes when our market closes, some bad or bearish news gets published or sometimes Chinese data on copper comes over the weekend which if negative can influence bearish sentiment of investors. So when this happens and the market opens on Monday after closing on Friday 12 am, we see this gap down opening in markets.
Plus on Monday morning other markets in the east like Japan and China open sooner than ours, so this news which came over the weekend effects them 1st thing in the morning and a trickle done effects begins and brings the price down worldwide, effecting Indian markets.