-- One of the main reasons of reuniting the Shield, apart from helping Roman Reigns' reactions with the audience, is to further fuel merchandise sales for the trio before the "inevitable split." Prior to tearing his triceps, Dean Ambrose was pencilled in as turning heel so that is likely still the plan.
-- In the interim, WWE needed opponents for The Shield which led to the Braun Strowman heel turn and him aligning with Drew McIntyre and Dolph Ziggler. While there appears to have been at least a little bit of consideration to putting Strowman with Finn Balor and perhaps Bobby Lashley to oppose the Shield, WWE turned Strowman because they are actually in dire need of a new top heel.
-- Kevin Owens has been downgraded and WWE has never been behind Ziggler as a top heel, so it made sense in that regards for Strowman to turn. It also allows the company to get out of Braun holding the briefcase since him failing to successfully cash it in as a heel is less damaging to his character than if he came up short while a babyface.
Source: Wrestling Observer Newsletter. Subscription information & prices available at f4wonline.com