If there is only one Siege Engine in play, and all conditions must be discarded, who decides the order? As far as I understand it, either they are discarded all at once (and Siege Engine cannot be used) or they are discarded in some arbitrary order. If Freeps chooses the order, Siege Engine goes first and cannot save itself. If Shadow chooses the order, some other machine gets to survive and Siege Engine is discarded per its game text.
They're discarded simultaneously.
Let's say you have 5 conditions in play, including
Siege Engine. Opponent plays
Sleep, Caradhras. At this point
Siege Engine triggers: one or more (5) conditions are
about to be discarded. As a response you can discard
Siege Engine to prevent that, saving the other 4 conditions.
In fact the simultaneous ruling is needed for
Siege Engine to work correctly. If conditions were discarded one at a time,
Siege Engine will "see" only one condition about to be discarded, meaning it can only ever protect one (since it'll be discarded as the cost before it has the chance to protect others). But because conditions are discarded simultaneously, it can protect the rest of the lot at one go.
In contrast
Scouring of the Shire does not discard itself as the cost. As the Free Peoples player, you get to choose the order in which to protect the conditions, triggering it once for each condition. (Incidentally, you can't trigger it more than once for the same condition, because after the first time, the "about to be discarded" situation for that particular card will be prevented.) It's only
after you're done responding that the game moves on; at this point the rest of the conditions are actually discarded.
With
SotS it's like having a bunch of conditions falling to the ground like leaves. You freeze time, place a support under some of them, and then resume time--the rest of the conditions drop, but the ones supported stay in play.