Spactory throws its spikes whenever it wants unless you have smart spikes as crosspath. This "random" behavior has impact the more track Spactory has to cover, so when placing it on bend with little track in its range, its piles will be more "focused". if thats opposite the pile will be Spread out.
This is why my belief is that most on-track projectiles follow "random" target priority, with exceptions like Guided Magic Wall of Fire which when bloon is in range the Wizard when about to cast spell it will cast it above the bloon based on its primary target priority (First - Strong).