On a 5-way helicopter jump, two of the skydivers tracked in the same direction and opened close to each other. The jumper who's filming reacted quickly, grabbed their rears and managed to avoid a mid-air collision.
In general, this jump plan seems to have been somewhat questionable. One instructor who viewed this clip noted that it appeared as if their dive plan was “Let’s exit, try and get together, and then track away and hope for the best.”
When everyone is off-level and separated like this, it can be hard to know where to track. (Which goes back to why there was some bad planning going on.)
Generally speaking, exits from a helicopter or a balloon are from lower altitudes and don't give you enough time to safely build up enough speed to move around, meet up with friends, and then track away. That's why some DZ’s limit group sizes that can jump together on helicopters and balloons. If this jump had just been 2 or 3 people, it would have been a lot easier to know everyone’s location and it would have been a lot easier to track accordingly to create separation.
While this jumper did check their air space before waving off and deploying, it doesn't appear that they checked their air space at break-off and may have tracked in the same direction as one of the other two jumpers who couldn't be seen in the frame at break-off. Or vice versa -- perhaps one of the other two jumpers out of frame tracked into this jumpers path and didn't check their air space below them.
Before deploying, some experienced jumpers suggest do a barrel roll so that they know exactly what is below, off to their sides, and above them. That additional situational awareness can tell a jumper to track for an extra second or two or take it a little lower for clear air space if need be. It can also give them information regarding where everyone else is so that, if they open up off heading, they’re not surprised when they see someone else in their vicinity.