Due to the print run of Justice League #1 Second Print being set before orders were due, DC received more orders than they had printed copies for, and had to put a halt to orders as they spilled over. Or indeed flooded over. As a result, DC will have to allocate copies to retailers, so no one will received the full amount they ordered, they’ll only get around a third of what they were expecting.
But in a quirk of fate, the third print will be back from the printers at around the same time as the second print. No I don’t know how that works. Either way, they will both ship for sale on September 21st.