Weird occurance while pvp-ing

A couple days ago when i was in the forest waiting for people to invade something weird happened. I walked back that little path by the bonfire to the little circle area where the trees wait to ambush, and i heard someone invade. Thats not unusual but when i got back out to the bonfire he was nowhere to be seen... I waited for a while and still nothing.... next thing i know someone is trying to cut through the fog door to get to me. They somehow spawned in the main forest instead of the area i was in.... Has this happened to anybody else? Does anybody know why it happened?

Isn't the area around the bonfire considered as the butterfly area? If so, you can get around that fog, both up and down to/from the bonfire, without going through the fog.
Or did the invader spawn in the Sif area, behind Artorias door? If so, the connection could have been established while you were in Sif's area, but the invasion happened after you left that area.

I've had a similar thing happen by the Hellkite Dragon. I'll be sat on the bonfire by the
Undead Merchant & I'll get up & climb the ladder, as I'm climbing the ladder fog gates will
pop up & I'll be stuck in the bonfire room with either an Invader down by the Undead
Merchant or above me by the Hellkite Dragon. And we have no access to each other
it's rather... Annoying. X

It's probably related to a bug in demons souls, where you could run in circles, back and forth across fog doors to empty boss arenas, and summon signs would appear outside the fog. You then had time to summon them and enter the boss arena before the phantom appeared, so they could also enter the boss arena. You could also summon reds and fight them inside the boss arena.

It means he connected to you before you entered the next area, even if your game didn't recognize it right away. Unless he took the jump to get back up there, how soon after the invasion did it happen?

So you were in that place where the ents pop out of the ground to ambush you when you pick up the loot (soul), right? They an invader spawned, and after a while he ended up on the moonlight butterfly side of fog gate, rather than the bonfire side?

Could be he fell! On his way toward the bonfire from the invader spawn point, he could have fallen off the cliff and survived, stranding him on the other side of the gate.