It's normal. Three weeks before our dd moved up to the 2-3yr olds room at nursery she suddenly began hitting the staff and other children. It came out of nowhere, she'd been at nursery for a year by then and had never done it before. She also started doing it at home.
The staff assured it was part of normal development and began using time-outs (which helped and which we used at home too). We continued re-enforcing good behaviour, encouraged empathy etc. Since starting in the older toddlers room we haven't had any instances (at home or nursery) and she's settled well.
It may just be a sign that your ds is ready to move to a more challenging environment and is becoming bored/frustrated in the current room. It's normal for toddlers to explore their boundaries.
And Jekyll/Hyde behaviour is completely normal in toddlers!
My 2.5 yr old DS goes to nursery full time as DP and I both work full time. Our DD is in school but she too went to the nursery, we know it very well and are very happy with the staff.
My DS is in the 18m-2.5 yr old room. Always a challenging one but he is finding it particularly difficult. The staff have commented that he can be such a wonderful, loving boy and then suddenly turn and be very aggressive and talk about (and doing) bashing people.
Yesterday he bit a member of staff and threw 2 chairs over.
We have discussed this at length and all of us looked for triggers (there are none that we can see that are obvious). They have a clear plan for dealing with him when he behaves like this and do say his behaviour should improve when he moves up to the next room (in a few weeks) as he is used to playing with older children.
I dread picking him up these days as I just don't know how to help him. We have tried all kinds of things - calm parenting, loads of praise, ignoring bad behaviour, sticker charts, talking about how he feels (his speech is excellent- nursery says way above the other children). He has never really settled in this room, although he likes going and adores the staff.
I am just seeking reassurance that he's not alone in his behaviour (and there's no magic way of dealing with him!) and that he will get better as he grows.