more info
They actually have SIX solid boosters. 2 big fat ones designated SRB-A. These are lit on the ground and are part of the original design of the H2A rocket. Four add-ons are air-lit. 2 at +10 sec and 2 at +20 sec.
perhaps the pad was not designed to take the exhaust of these add-ons? And the slight phasing of the start and burn-out of the two pairs is probably to avoid any interference/shock effects between adjacent nozzles if they start-up together.
There's a good page here, but much of it is in Japanese!