The jellyfish-like Psions act as support units, providing passive bonuses to the Vek. The Blast Psion is distinguished by its light grey coloration and bright orange cranial nodes. This Psion's presence causes other Vek to explode upon death, damaging adjacent tiles. The ability granted to the other Vek is called Explosive Decay.
Vek will not explode if Blast Psion also dies as part of the same attack resolution. E.g. if you push a regular Leaper into a 1HP Blast Psion, the attack will kill them both and the Leaper won't explode. However, in sequenced deaths where Blast Psion dies as part of series of other deaths, explosions will continue to happen until Blast Psion dies as part of a separate attack resolution. This reddit post has a video that shows how exactly this happens (notice the regular Firefly not exploding as it dies after Blast Psion's death is processed).
Vek explosions triggered by emerging Vek seem to be processed after all Vek have finished emerging from underground. This may lead to unplanned outcomes when an explosive Vek dies due to blocking a spawn point located adjacent to another spawn point. In this case, the Vek who emerges from the adjacent spawn point will be damaged by the explosion. Check this reddit post for more details and discussion.
If you kill a Vek with Explosive Decay by pushing it into a pit, the explosion will still damage adjacent tiles.