It doesn’t “cause it” in the sense that you must eat to gain weight.

The laws of thermodynamics will not be denied by a nap.

However, your passive caloric consumption is lessened at rest, so sleeping too much without changing your diet will absolutely lead to more fat being stored from the excess calories.

Think of it like this: if your current weight is stable, you are at caloric equilibrium. If you change that equation, in any way, you will eventually reach a new equilibrium.

You’re burning less calories? More will be stored (as fat).

You’re taking in less calories? Fat cells take calories to maintain. Excess fat will burned until a new equilibrium is reached.

But the equation that is the human metabolism is complicated. Cortisol causes weight gain by increasing appetite, aka, “stress eating.”

You very well might “lose weight by sleeping” if more sleep helps fix unhealthy eating habits by removing stress.

Tl;Dr Calories in=calories out is a fact of universal law. Where it gets complicated is managing appetite and passive burn.