thoughts and opinions on why ges whining at bed time all of a sudden..

My son is 17 months old now, We have switched him into a toddler bed bc we felt he was ready, hes always been a great sleeper and lays right down and goes to sleep when i lay him down. He was doing great, he love his bed he was laying right down and going to sleep and it was an easy transition, now its been about a month and the past 2 nights when i layed him down he whined when i left the room, he never did that before hes always been so good about just saying down and going to sleep. I hope this doesnt turn into a habit, what do you think the reason could be?

I thought the first night maybe he had a long nap earlier that day and just wasnt tired yet but everything was exactly the same day 2 and he still whined... he didnt for as long though.. i would just like some thoughts or opinions!