If you get ballistic missile at the phase change, the timing of allagan field for the last sets of towers is fucked, because of the extra 5 seconds which are taken by ballistic missle mechanice until field is cast but towers spawning anyway.
Thats why you want to push into last phase ASAP, thus the timing of field and towers is properly aligned. At what %HP does the phase change occur, 35% or 40%?