TPS65131 inverting dc-dc overshoot

I am trying to make a ±12 V power supply with a TPS65131 DC-DC converter.

The positive rail looks just fine, but the negative rail overshoots to around -12.7 V and then slowly recovers to -12 V, then the process repeats. Increasing the output capacitors just slows down the process. Here is the oscillogramm with a 330 μF capacitor:

The schematic I am using is pretty much the same as the datasheet, so I assume that the problem is with the PCB.

Here are the schematic and the PCB:

1. What causes this behaviour? I understand that my PCB layout is nothing like the datasheet recommends, but I am interested in the reason.
2. What is the problem with my PCB exactly?
3. Can it be fixed without layout changes, with component changes?