So far, my understanding is that March rumors are based on timing from previous years.
Recent rumors suggest supply chain problems. Apple is having a lot of those (e.g. AirPods). The products may be ready, but production isn't. With the iPad sales slumping, they will want to get products in peoples' hands quickly. Last quarter they under produced iPads, likely do to expectations of people holding off for 2017 models. They could have sold more.
https://www.macrumors.com/2017/02/24/10-5-ipad-12-9-ipad-may-june/
The "goldilocks" 10.5" iPad it likely expected to sell well (only time will tell, but you have to make projections). The original 12.9" was too big for many, but a larger screen on a 9.7" frame may have an iPhone 6 effect (a good enough reason to upgrade).
tl;dr
There are supply issues, a predicted high demand, and pressure to turn around iPads sales. So, March is desirable, but may be too soon.