All my previous flights were daytime (about 20 flights) and everything went fine so I tried my 1st night flight yesterday. The day before this, I did do a IMU calibration and then at the flight site I also did a compass calibration. Everything went normal. So after a normal through preflight, I took off and the flight went perfectly normal. The problems started when I go to land. I always do manual landing so I can set her down very gently. But this time when I got down to about 2 feet , the P4 started bouncing up and down and it just didn't want to land. I pushed it up to 6 feet and hovered (it was steady), while I thought about what to do. I switched the RC to F mode and tried again with the same result. So now I figured it was the VPS going crazy, so I go back to P and went through with the landing. With it bouncing, I did my best to set it down at which it ended up on it's side. Left stick down would not cut power after it was on the ground, so I get warnings of motor rotation blocked (something like that) which is because the P4 was on it's side and 2 props were jammed, the other 2 spinning. I finally did a CSC and shut it down. No damage to anything as I landed on grass. Now this log is the second flight which I did right after. It is short because I just took it off and quickly landed just to test again. And the P4 acted the same way. Upon landing it also fell on its side. Please give me your thoughts why this all happened. This is my first crash (sort of) and I really need to figure this out. My guess is the VPS system took a dump for some reason, although I also see negative altitude numbers all of a sudden. Why is that??