why this happens?

[mach1]

at around minute 1.30

http://www.youtube.com/watch?v=28nuXwQyGa4&feature=youtu.be

i guess its probably because the parachutes are placed below the center of mass but there are 4 of them, the lander should not flip that way.

[Vanamonde]

Your parachute attachment points are below the center of mass. Once the ship is hanging from the chutes, the body of the ship is acting like a pendulum. You were fortunate that ASAS and RCS were strong enough to right the ship again before landing, and to hold it that way against the pendulum effect.