Clips out of sync

Please help me sort out this issue. Please see the image, tell me why this happens and how to aviod it. It happens to me many times. First I edited these clips at the end of the time line,  and when a edited another fiew clips at the start of time line, all these clips have got jammed. Though could sort out them one by one, it happenes every time when I edit another clip on the timeline, please explain how avoid this!