Scientists theorize that Australia was once part of a larger mainland area but due to plate tectonics, broke away. According to island theory, scientists can explain ALL BUT one of these occurrences as a result of Australia becoming and staying an island for a very long time. That is

A large assortment of plant dispersal mechanisms.

Explanation:

Pretty much everything can be explained about Australia, as to where it was in the past, with which other large landmasses it was connected, its flora, its isolation from the other continents... One thing that still needs explaining is the large assortment of plant dispersal mechanisms, which is not typical for well established islands.

The reason as to why this is weird and hard to explain is because the well established islands have flora that is well adapted to their living conditions, and also it lacks competition from outside. In other words, they rule the place. The plants in Australia though exhibit dispersal mechanisms that are typical for the plants of the continental masses that are connected, so it causes big problem in explaining it.