Terraria internally only supports a total of 200 npcs/enemies being active at a time, presumably to put a cap on performance issues. If a new enemy spawns while there are already 200 other enemies alive, it gets spawned into an "overflow" slot, to make programming easier and eliminate edge cases. When a boss gets spawned, the game simply displays the "... has awoken!" message for which ever enemy is in the slot that was selected. The key is that not all enemies will actually override this slot, so in this case a boss was attempted to be spawned but failed without changing the overflow slot, which still contained some other random enemy, resulting in these messages.