There is no one planting them; it is the natural power of the urban landscape asserting itself. It seems like there ought to be trees in that spot, and so there are trees in that spot.

Or possibly someone is building a Lego model of your landscape and had some spare trees - and in a strange Voodoo-like sympathy your landscape has been forced to respond.

Or maybe moles are building ladders to the stars.

Possibly.