Wheels changing on Giant Rebel 4
What is going on with your wheels is known as being out of true, or in its extreme form (like the photo that you provided), taco-ed. This is not all that uncommon on entry level bikes like the Giant you have. 

This can be caused by several different things - the most common is just general wear on the wheel. Over time, small changes in pressure on the spokes cause the tension around the wheel to become uneven, which causes the rim to warp. Other causes include poor construction of the wheel, cheap parts, or abuse. 

If you have fixed your wheel several times, and it still happens, then it is likely a defect - I would take it to the shop you bought it from and ask to see if they can look into a warranty for you. I don't know Giant's warranty policy off the top of my head, but I have worked at a shop that sold Giants, and IIRC, they were generally accepting of defective stuff. Some bike shops will do everything in their power to not take in your wheel and attempt to get a warranty for it, but if you persist long enough, they will probably make a phone call or two.