https://www.cnet.com/news/apple-forces-oneplus-to-move-its-oneplus-6t-launch-up-a-day/

There's a lot riding on the OnePlus 6T, which marks a potentially major turning point for the company. The phone will be sold exclusively through T-Mobile, its first partnership with a US carrier. The deal could catapult the company into the mainstream. It'll also be the first mainstream smartphone with an in-display fingerprint reader to hit the US market. 

So it makes sense that the company doesn't want to get overshadowed by Apple, which is expected to unveil iPads and Macs at its event in Brooklyn.