Paying attention to swell forecast when anchoring
Notice that there isn’t any wind to speak of. We don’t know the complete situation but it’s safe to assume that the anchor was not really dug in, the swell changed the direction that the boat was lying and there wasn’t enough scope to hold the repetitive tug of the swell. Note that, according to Bild.de who published the pictures, the following sequence took place within 20 minutes. The set of waves are really not that big, a strong testament to the power of even a small swell.
Check the surfing websites for the area -there are loads of free ones out there and they are usually very accurate. www.magicseaweed.com or www.windguru.cz both show swell forecasts clearly and it’s relationship to the wind in most popular cruising grounds.