Affray is an either way offence, meaning it can be dealt with either at the Crown Court or at the magistrates� court.
Where it is heard and sentenced depends upon the circumstances of the individual offence and in making the decision whether to accept jurisdiction the magistrates (where all cases are heard initially) accept the facts of the offence as presented by the prosecution.
Their guidelines suggest that where the offence involves �...a weapon or throwing objects with conduct causing risk of serious injury� the matter should be committed to the Crown Court, which it seems they have done. You are now going to Crown Court for a Newton hearing because (presumably) although pleading guilty, you are disputing the facts of the offence as presented by the prosecution.
Magistrates� sentencing powers for the offence are limited to six months custody and in committing you to the Crown Court (based on the facts from the prosecution) they believe your offence warrants more than that.
If the Newton hearing goes against you it is therefore likely that a custodial sentence of at least six months will be imposed. Your guilty plea will possibly reduce this, but your previous record of offences involving violence (particularly the ABH four years ago) will not go in your favour.