The Wiki states, "A cactus destroys any item that come into contact with it, including other cactus in item form. Falling blocks affected by gravity such as sand and gravel are also destroyed when falling onto cactus, but cactus does not destroy falling cactus blocks; instead, the falling cactus transforms into items upon touching a cactus."
I increased my tick timer to 300 and many of the falling cactus blocks are being randomly destroyed.  I have included a screen cap. of the simple test farm.