This is the 5547th most frequent Japanese word.
かぶさる
"かぶさる" means "to hang over, cover, or overlap."
In this sentence, 'かぶさる' conveys the sense of something extending over a surface in an enveloping manner.
雲が山にかぶさった。
The clouds covered the mountain.
Here, 'かぶさる' is used to describe a blanket draped or spread over someone, indicating a protective covering.
毛布が赤ちゃんにかぶさっている。
The blanket is covering the baby.
'かぶさる' in this context illustrates the accumulation of leaves to the point they overlay a particular area, showing layering or accumulation.