youcannotbeserious you are lucky in that your ex is normal.
Mine most certainly is not and I know, it sounds mad, but I know for a fact that in his case, the keeping of stuff there, was controlling. I say 'was' because he doesn't do it any more, because luckily for the kids, his now wife is sensible and she tells him he isn't on when he tries to make them leave stuff. Honestly, they had to leave
I'll tell you why its controlling. Anything the kids used to get given from him or his side of the family, had to be kept there. Clothes, toys, anything. He used to see them twice a month. So, all this stuff didn't get used in between. If the kids wanted to bring a toy back, that they had been given for birthday or whatever on a visit to him, he used to make them leave it there. And they might have just been given it, and wanted to play with it obviously as it was new, and they wouldn't be able to for 2 weeks!! Why not let them bring it home, and take it back there next time?
Once, ds took an action man thing there and was playing with it all the time he was there. On leaving to come home, his dad talked him into keeping it there so 'he would be able to always play with it when he went there' wtf??? Erm he could always play with it when he went there anyway because he would take it there if he wanted to, and that way he could play with it here for the 2 weeks until he'd be seeing his dad again? Ds was upset and only got it back cos I rang dickhead to tell him he needed to bring it the next day when he was visiting round here.
Now, things are as they should be. If anything from there like clothes come back here on the kids, I will wash and send back, and they do the same.
He stopped doing it really, when I stopped confronting him. I made out i didn't care any more (of course I did care, I cared a lot, especially when I had upset kids to deal with because once again he had made them leave something they wanted to bring home)
It was almost as if he completely cut out of his mind that they had a life away from the 2 visits a month with him!