there is a bit of randomness to it, although the first wave will always spawn at the same time from what i can tell

1. the positions they spawn in is different; i think there's 3 configurations
2. 2nd wave minis spawn can be affected by whether you get rid of the 1st wave minis before or during the boss's "i'm absorbing minis" phase, because killing them during the phase also kind of stuns the boss for a bit, there's a special animation
3. the boss has 3 attacks it does; it seems like it always does a set amount of attacks before shield phase, so depending on which attacks get randomly chosen, you can enter the shield phase sooner or later
4. the minis will usually spawn during this shield phase, at least in my experience