And I do know why, or at least, what the catalyst was. We just moved house two weeks ago and while we're very much in the same area, he sees all the same friends, goes to the same groups, shops etc, the house is Different and his bedroom is larger, has different shadows and he has suddenly become almost phobic about going down. 
Bedtime in the old house used to be - bathtime, mostly with DH, teeth, then change into PJs in bedroom, say goodnight to his stuffed animal friends, and a bottle of milk and a cuddle while he's drinking it. Put down pretty soon with teddy, sit in there a few minutes, then I'd pat his back, say goodnight and leave. He'd rarely call us back and would go to sleep on his own. (He was breastfed to sleep until last christmas and then we started with the bottle.)
Now, he starts racking up the aggro at bathtime (everything is Different and Wrong) and by the time he's upstairs he's already pretty tense and getting hysterical. By the time we get his PJs on, he's upset, not wanting to say goodnight to his friends properly, and then won't sit still on a lap while he's having milk. He then starts screaming the instant we get up to put him in the cot (the sides are still up) and says 'things like 'I don't want to go in there!' and says the room is spooky, there are Monster Eyes, Bear Shadows etc. We never had any of this before. I've eradicated as much of that as I can, with a blackout blind and by making sure landing lights are off, and we've gone in there during the day to tell the silly shadows to Go Away, but he's not a rational being when he's tired and scared!
The only way I've managed to get him to go to sleep quickly and without hysteria is by lying down with him on our bed. Takes a couple of minutes. I personally don't have much of a problem with this, but I think DH is worried it'll make problems in the future. Also, we are due our second baby in about 6 weeks, and while DS is looking forward to meeting his new brother, I know it's another disruptive thing at the moment and almost certainly causing him stress.
WWYD? We're planning to co-sleep some of the time with the new baby, and in any case, he'll probably be put down in our bedroom of an evening, so I'm concerned that DS1 needing to be laid down on our bed too will be a problem. Anyone done this sort of musical beds thing with a toddler and not had it last for years and years? 