It seems like the undead are invading everywhere and Bonemass is trying to take over by beating the other bosses.
– The undead are hostile to all wildlife and battlers, though not all factions are hostile to all others (Trolls and Greys don't molest deer & boar, for example).
– The undead are present in most biomes; skeletons in the Black Forest Burial Chambers and strongholds, Draugr villages in the more distant Meadows, Skeletons and Draugr garrisoned in Mountain strongholds, and the vegvisir located in these areas are sources for the locations of The Elder and Moder. They are gathering intelligence about the other bosses.
– Bonemass can resist the damage the others can deal, and his remains have the power to locate the hidden treasures in other biomes.
He has the power and motive to knock the others off, and his underlings have demonstrated the desire to clear the way.