Different teachers do have different ways of dealing with children.
It could be that the "nice" teacher takes him at times when he is fine, the other teacher having to do things he finds hard to control himself during (eg "nice" taking mornings only and he gets worse as the day goes on) I found ds got worst on Thursdays, so that teacher had the brunt in year R (job share).
It could be the nice teacher is better at dealing with him, or he relates better to them. Doesn't necessarily equate to the teacher he likes better.
It could be a personality clash and their personalities rub up the wrong way so as soon as she asks him to do something he acts up because it's her.
It could be that she's telling it as it is, and the other teacher tends to avoid confrontation. I discovered at preschool that they weren't always telling me when ds'd played up because they could see it was stressing me out. Very kind, but not helpful in the long term. This year's teacher I know tells me as it is, so when she said to me "he's improving rapidly" I know he is and could have kissed her which is actually a great thing to be confident she's able to say that.
It could be that they have a policy of not discussing such things "at the door" so to speak, and wouldn't tell you until they had a meeting planned. Not necessarily helpful, but some parents would get very upset if they thought things like that were discussed at the door where others can hear.
It could be that one of the teachers is more experienced (could be either) and they can see the way it's going (either way) where the other one can't.
It could be the SEN teacher, because that's what she does, spots SEN in all sorts of children. If you look for it, you can see it iyswim.
It could be that they thought he was doing low level behavioural issues, and had improved, and then something during the temper made them think "actually that's not right".
Or it could be that the "nasty" teacher just doesn't like him and has taken against him and will always see him as a problem.
I doubt that's the case, because she would then be saying he's naughty etc. not he needs help from SENCO.
Go to the meeting with an open mind. Don't come at everything they say with an excuse or "he doesn't do that for me". Listen to what they say. If you're still concerned at the end, speak to the other teacher, last year's teacher, and possibly the head. See if you can get a picture all round.