My physics education was shit, so I can't offer formulas*, but I remember this much: Objects maintain their velocity unless impacted by a force. In this case, the velocity is effectively 0 since the leaves are at rest. Gravity doesn't act very strongly on them either since they have such low mass. Hence their momentum is low enough for them to kind of sit in the air for a bit. If they were wet, they'd follow the net much more closely.
Same reason you should always wear a seatbelt.
*or correct orthography...
