Here is the scenario:
My Familiar has a strength of two and can therefor carry 7lbs. (Str score X15 divided by 4 for being tiny (p176 PHB) )
I cast Reduce on a rock that weighs 56lbs making it weigh 7lbs.
I have my familiar fly over a target. Then as an action I cast Enlarge on the rock, cancelling out the reduce spell and enlarging the rock x8 making it weigh 448lbs. This is too much for the familiar to hold and it drops the rock. Not as an attack, but because it has no choice.
An alternative but similar thought for comparison. If I use an action to dismiss the familiar does it drop the rock? (ending concentration on the reduce spell and still dropping a 56lbs rock)
Does this count as an attack from the familiar, or because I used an action to cause it, is it my attack?