Kobolds don't have [SIEGER] tag.
Goblins don't either, but they still send "sieges." The sieger tag actually makes them perform an extended siege. Without it, they spawn and immediately bumrush you. With it, they sit and make camp for a month or two before bumrushing you. Only humans and dwarves have the [SIEGER] tag.
Similarly, the [AMBUSHER] tag makes all attacking forces a civ sends begin in ambush. They'll still send ambushes in ambush without it, but with it sieging forces also start in ambush. Combining the [AMBUSHER] and [SIEGER] tags results in enemies that sit hidden at the edge of the map, with the only sign of their presence being a campfire, which can be pretty easy to overlook. It's actually kinda terrifying because you could go a whole month past the expected due date of the next siege and make the assumption that this season's gonna be perfectly safe so it's a-okay to let the dwarves out to do whatever, before getting ambushed by multiple siege-platoon-sized groups.
The [SKULKING] tag that acetech mentioned causes them to send attacks based upon thieves' success rates. More success leads to more attacks. the [ITEM_THIEF] tag actually makes them send thieves to steal your items - and makes them hostile to all other entities by default. ([BABYSNATCHER] makes them send thieves to kidnap youths, like goblins.)
Kobolds have [AMBUSHER], [ITEM_THIEF], and [SKULKING]. This means you'll rarely see sieges unless their thieves just keep taking all your crap and you keep making it easier on them, but that when they do send a siege, you won't be ready for it unless you're watching the clock/paranoid 100% of the time.