Hello

It will be more helpful to us if you would upload a photo of the leaves that have fallen and the entire tree, so we can better understand and diagnosis what may be going on.

thanks

The common name of the insect that causes this is the maple petiole borer.  The adult is a very small wasp in a group called sawflies.  None of the sawflies can sting, so at least you don't have to worry about that.  The adult sawfly inserts a tiny egg into the petiole of maple leaves (mostly sugar maple and Norway maple) while the leaves are very young and tender.  A very small grub-like caterpillar hatches from the egg and feeds inside the petiole.  At the point of feeding the petiole discolors and the leaf usually falls off of the tree.  The grub finishes development in the fallen petioles.  In over 30 years of advising people on this insect, I have never seen a case in which the number of leaves that are lost was significant to the health of the tree.  It looks like a lot of leaves sometimes (as in your photo) but it is really only a fraction of the total number of leaves on the tree.  The damage can be ignored, no controls are called for.  Insecticides could be applied, but the timing is difficult (sprays would need to go on well before you see any injury or fallen leaves) and you would likely kill off a good number of beneficial insects in the process.  Raking up leaves won't help reduce the future incidence of this insect unless they are thoroughly shredded, burnt, or buried to kill the grubs or keep new adults from emerging from the old leaves.