I think you are confusing cause and effect.
Contained pressure is what splits the rock.
Only after it is split can the contained pressure be released through the new opening(s) producing an explosion.
Watch Mount St Helens footage, the whole mountain heaves upward, one side falls away uncorking the volcano. The pressure below can then be released. In that case in a severely explosive manner.
The danger with La Palma is the island is already showing signs of being unstable.
There is a fissure running North South right where this volcano is going off.
The fear is that the eruption and disturbance associated with it could destabilize the rock further causing half the island to collapse into the ocean. That would be the cause of the tidal wave.
Watch the video it will help - less than 5 minutes