The clan invasion had the Lyran military in a bind. With heavy mechs they could match the clan's firepower advantage, but only at the cost of mobility. The demand for larger numbers of fast medium mechs from the field was obviously not a viable solution - such mechs could not stand up to clan machines.
But something had to be done about the enemies outmaneuvering the Lyran armies. The answer came from the DCMS - an area denial BA. A unit that could quickly be deployed to cut of the enemy's line of advance allowing the heavy units to engage him.
Several alternatives were offered, but in the end some sneaky maneuvering by a cabal of lower-ranked officers managed to get a small (heavy-weight rather than assault) unit selected as the winner. The top brass was rather shocked when the finally discovered not only had a unit lighter than the Kanazuchi been approved, it had also been equipped with weapons not part of the initial specifications!
Aggravator M3 Multipurpose Mobile Minefield
IS heavy BA
Chassis 300 kg 12 slots
Motive system 1 ground MP 0 kg
Armored glove RA 0 kg
Heavy battle claw LA 20 kg
Basic stealth armor 10 points 550 kg 3 slots
LRM1 Body 11 rounds 155 kg 5 slots
SRM4 LA 4 rounds 400 kg 3 slots
LMG RA 50 rounds 75 kg 1 slot
Cost 403,500/1,614,000 BV 60/312
So, the logic: Area denial BAs like the Kanazuchi are just targets if the enemy doesn't move into their engagement range. In addition, if you just want the enemy to stay away from a location you don't really need a lot of endurance, you just want a substantial punch (thought some endurance is good to have, thus the LMG). And of course you want the BA to be easily transportable so that you can deploy it quickly.
Thus: A LRM tube with lots of ammo to keep the enemy honest - you're not really expecting to kill anything, but it means the enemy can't just park at 12 hexes and laugh at you (and you might get lucky!). A (relatively) large SRM rack with a couple of rounds if anyone actually does close with you. And finally no more than heavy size so that it can ride omnis.