When my chain is on the biggest gear in front and biggest gear or second biggest gear in back, my chain slipped out of the front gear each time I pedal back. Could someone please advise why does this happen?and How can I fix this Thanks

First off all you shouldn't be shifting to the extremes. If your in the largest ring in the front I usually go 4 or higher in the back with a 7 speed cog. Thats probably normal, when I'm in 2 in the front and 1 in the back my chain will jump down a cog on the back when I pedal backwards. Likewise if your in the smallest ring up front and the smallest out back you might have problems. Hope that helps

It means your chainline is off and a shorter bottom bracket spindle will improve the situation if one can be fitted. However, as dr. raleigh is right and you should avoid the big-big combinations.

Also, backpedaling isn't a problem as there is no reason to do so when riding unless the chain spilling is so severe it occurs when you back pedal the fraction of a turn need to bring the pedals into position for a standing start.