As was said above, it's not a glitch, but a consequence of the way location-specific missions work. Basically when you start one of these missions you are put in a separate instance, in which a limited number of other players can join you. This happens transparently, but is actually visible fairly easily if you're in a group: if you enter one of these missions and other people in the group don't, you'll see them as being in a different phase.
So when you're doing the Kenn farm mission, you're actually in your own little "Kenn farm mission instance". Sieges don't override these instances (which makes sense, it would be annoying to have your mission progress lost because a Siege just popped up), so you need to stay out of the area to see them.
Sieges are not random at all or so I've been told. They happen at certain intervals, like I think once an hour. If you do something in the game, like play a coop, pvp. or keep longing off and on, you keep going into different phases. You might keep entering phases where a siege just happened and reset the timer for you. You need to stay in one phase or if and when it's possible join a friend that has a siege happening.
Sieges will be easy to see-the marker will who up on your map just like the arkfalls do, but it's a different marker. It's blue and when you get near the siege, the area will be surrounded by a large blue circle like the arkfalls have a big red one. And all sieges happen at the same time. So, you will have 4 choices of where to start.
sieges are phased, so if you are in the area when one begins it will not show to you so just play as normal and look on your minimap for the green logo to tell you one has started.
My bf and I were playing yesterday, at Kenn farm I went afk and he continued to play, when i returned he was phased at Kenn farm for the siege (having left the area and returned) but i could not see anything. I had to fast travel somewhere else and then teleport to him in order to see it.
I've noticed that arkfalls (usually single and not major) occur near a siege that will take place within say 5-10 minutes after the finished arkfall. Not sure if this always the case. Just an observation really. Also, a tip for advancing sieges if your the first few there is not to cure the objective points right away. Knock them down a bit and wait till more players come to help out. I believe the siege won't start the timer until all the objective points are captured. Hopefully I'm not totally full of shtako.
Also the events are phased, if you are in the farm doing the kill the bugs event you are phased there, not the siege when it pops. This happend to me on the Bolanis area one; I saw the maker on the map but no siege. I had to run out of the area and back it to see the correct phase.