Worker rights and financial/business regulations
Tin foil hat time:
Tories wanted to reduce workers rights, be more like the US labour market where it is easy to hire and fire. Couldn't do it because of EU workers protections
Cameron announces referendum.
Referendum voted leave with the hopes to get more control back, and less immigration
Tories know they need access to the single market. They also know the EU is unlikely to allow access without similar rules to what Norway has - which means freedom of movement of population. Basically what most leave voters don't want.
Tories sign agreement because it's our only option.
They are then free to fuck over workers rights like they wanted to, so the poor and unwashed masses that voted to leave get less protection from business and still no real controls over migration