The way NI contributions are paid and collected changed from April 2015 - you should have had a letter from HMRC, I did, but it didn't make things much clearer to be honest.
Here is the wording from the proposal paper:-
Detailed proposal
Operative date
The measure will have effect from April 2015. Class 2 NICs will be collected through SA from
April 2016, for the 2015-16 tax year onwards. The current six monthly billing system will
cease from April 2015.
Here is the current page from the HMRC website which explains why but no real details on how..
https://www.gov.uk/self-employed-national-insurance-rates
I'm assuming the amount required based on profits for the previous year will be added to the amount of Incoming Tax outstanding and you would pay it in one go like your tax. (I did see somewhere there would be an opportunity to pay in advance by installments if you wanted too, which is no different to the DD really)