As has already been suggested, the obviously very low noseweight is most likely to be the cause of its instability. Whereas UK 'vans often suffer from excessive noseweight, Continental caravans, like yours, often have inherently low noseweight when empty. Either way, you'll have to adjust to suit, if necessary, either by removing unnecessary weight up front (typically UK 'vans) or adding ballast ahead of the axle (typically Continental ones).