Comparing explosive properties of mercury(II) cyanate and mercury(II) fulminate
This can be attributed to the formal charges of fulminate ion and cyanate ion. Structural stability of a compound sometimes depend on formal charge.


We observe that formal charges of fulminate ion is higher than cyanate ion for which fulminate ions are said to have a less favorable molecular configurations which leads to facile decomposition of the fulminate ion.
Fulminate vs cyanate stability has been demonstrated by using their corresponding silver compounds. Here is a video demonstration.