My younger brother did that when he was 2 years old and moved from the baby bed to his own big boy bed but it only lasted 4 months.
Our mom just put down blankets for him near the door and a pillow. I guess he was just more comfortable there on the floor instead of the bed and he turned out okay and he's 23 years old now.
Every time mom or dad would pick him up and place him back on his bed he would always be back on the floor by bedroom door in the morning.
It was either he was more comfortable there or maybe sleep walking during the night and then crashing out there or he was getting up in the middle of the night and wandering around the bedroom and then just crashed out there from being too tired.
I wouldn't worry about it too much and just let him or her sleep on the floor until they decide they want to sleep on the bed.