Look at the code at RT-19739 with setWrapText(true) uncommented.

The content bias of the label is HORIZONTAL, which means the height depends on it's width, but BorderPane will grow the width of the child to the maximum. The correct behaviour should probably detect if the node will fit into the height with the current width and grow the node only if it doesn't.
There are also some strange no-op computations in BorderPane layout which should be investigated.