It's possible that your ex's legal team are questioning specific facts which the judge might need to consider when determining the sentence and therefore calling for a 'Newton trial' to examine those facts (with the judge, in turn, seeking advice as to whether such a trial is actually necessary).
For example, someone could be charged with ABH, with the prosecution saying that he launched a premeditated and sustained attack using a weapon. The defendant might say 'Yes, I'm pleading guilty because I did cause actual bodily harm to the victim but it came about from a single blow, without a weapon and which wasn't pre-meditated'.
If the judge were to accept the prosecution's version of events (without any evidence for them being put forward) the defendant would almost certainly receive a lengthy prison sentence for the offence. If he simply accepted the offender's version then he might feel compelled to pass a non-custodial sentence. So, even though there's no doubt about the offender's guilt, a trial still needs to be held to establish the facts upon which sentencing is to be based.