Hello,
As usual, beginners make unexpected actions 
Here my findings you can easily reproduce.
Unexpected behaviour when inserting a video on the time line.
In order to reproduce the problem, set on the time line of a new project 4 videos (in sequence).
Add image and sound on the second video and audio track above the last clip (not aligned).
Group the two last clips with their added image (and sound).
Above the second and third clip add a video on track 2.
Add some space between clip 2 and clip 3, smaller than the above video.
Drag that video on track 1, in this space.
Work around : provide space greater than clip to insert.
Sorry 