There is just a max limit of stuff that can be spawned at once. You'll generally only run into the issue if you use the purple resource spawners towers to farm up a ton of resources before the wave starts.

If I had to guess, there is a limit due to performance reasons. Perhaps on some of the levels where you defend a moving objective there are so many turrets placed that become irrelevant later on in the level as the defense objective leaves them behind that they decided to implemented a maximum limit of turrets.