I have to unplug the power cable. why?

My daughter's iphone 4 had a bad power flex cable, so I replaced it.After replacing it I checked to see if it was working,it was so I turned the phone off to finish putting the back cover on. When I tried to turn the phone on using the power button, it would not work. I took the phone back apart, unplugged the power flex cable, plugged it back in and it started working properly again. It also does this when I hook it up to a charger. Can anyone tell me what's wrong?