Hi mdeni,
This is more so a short coming of the cascade previewer. The reason this occurs is because the bounding box for the particle system is attempting to adapt in real time in the viewport. You can see this when you enable the bounds in the viewer. As soon as the system exceeds the bounds it recalculates and during this time the particles will shoot off quickly until the bounds are done adjusting.
I logged the ticket here: Unreal Engine Issues and Bug Tracker (UE-41681)
Keep in mind that there is a good chance this won’t be addressed quickly.
Cheers,
Ed