Well bots love to choose a certain way, if it has a CT or T important infront of the way they should go.
Like dust2, where CT Start there is two ways out, and both ways should have a CT important before the exists. Also their spawn waypoints could be to close or not enough of them.
I experienced the pileup issue in de_ancient CT's kept piling up in CT Spawn, so I added more CT Importants and waypoints around and in their spawn area. The issue was solved
