This is versus the usual timeframe that we have come to expect from Apple, which is where mass production typically takes place in August/September ahead of the iPhone’s announcement and launch. According to Kuo, the reason behind the delay is due to production difficulties.
The rumors are saying that Apple’s iPhone 8 will be packing a ton of new hardware features such as OLED display, 3D laser cameras, cameras for AR/VR, and also wireless charging. Apparently these “cutting-edge” components which are custom-ordered are harder to manufacture than expected, which is why the handset’s mass production could be delayed.
Note that this is only for the iPhone 8, or iPhone Edition as some are calling it. Apple is expected to launch three models this year: the iPhone 7s, 7s Plus, and the iPhone 8, so presumably things should proceed as per usual with the “s” models, but either way take it with a grain of salt for now, but at the same time if you are considering the iPhone 8, either move quickly or be prepared to wait until next year.