Could be many factors:
Not enough cash to fund the next phase of development. Lots of developrs go through cash-flow problems from time to time, which slows development.
Planning Conditions being discharged. This can take some time and some Conditions are more problematic than others.
Maybe they've altered the design part-way through the build so had to re-submit a revised Planning Application.
Workers not being available
Lack of potential buyers. So the build rate would be slowed until the market picked-up. This is because Council Tax on the completed buildings would have to be paid by the developer until the houses are sold.
Maybe they've hit a 'snag' with something they've dug up on site?
I doubt anyone is going to tell though
