This is a terrorist's work accident that led to the killing of 7 and the wounding of others.
The terrorist, Abu Omar Kadum, placed a large charge on one of the gates of the fence, lit the fuse and went back.
The charge did not explode and Abu Omar returned and lit the fuse again, but again the charge did not explode.
Abu Omar Kadum approached the luggage on the third, decided that something was wrong with it and took the luggage north with him and a member of the group that was standing there.
*North = on the fence line towards the north.
Near the arrival of Abu Omar Kedom to the group, the bomb exploded and hit everyone in the area (dead, scattered, injured).
The reporter points out that the detail brought up and the testimony of what happened is from one of the Hamas members who were in constant observation and surveillance of the fence.
In an addendum to the testimony, the reporter states that no shooting was fired by the IDF at that group, nor at the medical forces that arrived at the scene and evacuated all the bodies and the wounded.
Source - Eran Malka
Related article: UPDATE from Gaza border: IDF releases video of deadly explosion