Impossible Round of Invasion Onslaught

It seems that a lot of the levels on Invasion depend on some kind of RNG. In an Invasion Onslaught game i played, one of the rounds was a round where you had to stay in the circle and navigate the map. Usually this isn't too much of a problem, but in this particular instance of the round, around 20 vel'koz spawned at the END of the map INSTANTLY and immediately started shooting. This was from a part of the map we couldn't even access until the very end, and it created so many projectiles it was basically undodgable, and a horde of rek'sai and skarner to go along with it. Is it just like this, or is it unlucky RNG, or what? Either way, it was ridiculous and completely unfair.