There could be a common cause (all 3 cables terminate in the same city, could follow a similar path where a single event/failure occurred, etc.).
Another possibility is that these cables provided an ideal training environment to test the capabilities of an adversary to cut off a region from the rest of the world. I'd think an adversary in wartime would cut all 5 cables. Vietnam could be the victim of an aggressor who just wants to practice/prove capabilities without much fear of repercussions.
I don't know who would benefit from crippling Vietnam's internet infrastructure today, but it could still benefit another nation to see/prove that it could.
Another possibility is that these cables provided an ideal training environment to test the capabilities of an adversary to cut off a region from the rest of the world. I'd think an adversary in wartime would cut all 5 cables. Vietnam could be the victim of an aggressor who just wants to practice/prove capabilities without much fear of repercussions.
I don't know who would benefit from crippling Vietnam's internet infrastructure today, but it could still benefit another nation to see/prove that it could.