It dries out, loses volume and weight, then deforms and tears. There was a dike near Wilnis that actually broke in 2003, leading to half a metre of water in some streets there. Maybe the dike that was in the news a few weeks ago (it didn't quite break, but there was some danger) was a different one; I didn't check, but it was also near that village.
Interesting! Why would a dike be under greater threat in a drought when there is less water?