Because hairs usually stand upright, the particle system is doing what it is supposed to do.
In the 3D Viewport of the image below you can see a similar setup to yours. Some of the planes in my collection are rotated though, but I did not Apply the rotation.
In the second Viewport I did apply the rotation.
Object > Apply > rotation.
Note how the upright planes in my collection (blue) are flat as particles, and the flat planes in my collection (orange) stand upright as particles.
You can rotate the leafs in your collection so they will stand upright, and then apply the rotation, so the leaf particles will be flat. Like the blue planes in the bottom picture.
If you want the leafs to be higher from the ground (because they might still clip through the ground plane) then you can move the individual meshes of the leafs away from their origins in Edit Mode, as seen in this (3rd) image below.
It wasn't apparent to me when I first answered this question (i think someone may have hinted to it in a comment but not clearly enough I'm afraid), but there is another, if not a better way to do this. It is a bit hidden though.
In Particle Properties check the box Advanced, and then the box Rotation. Under Rotation you can change the particle Orientation Axis.