all motors start at the same speed except one motor at pin 6 (PORTD |= B11110000) keeps on increasing infinitely even if the throttle is at low and results into bumping of copter. I lost one propeller due to that.
Are you sure that you have the sensors aligned properly with the orientation of the quad ?
If not then when the software attempts to keep the quad level it will increase the speed of the wrong prop, that tips the quad further out of level, the prop speed increases in an attempts to correct it, the quad tips further...
I am sure that you can see that this does not end well.