Are there other reason for the loss of the megafauna...

The main reasons as to how we lost the megafauna are as follows:

Human Impact: We needed a lot of land to support our lifestyles over the years, so many forest got deforest. None of the forest revived so native animals and plants that live there lost their habitat. The others are hunting or excessive burning off in the grass lands.
One of the biggest causes of the extinction of megafauna was because of when the humans arrived around 48,000-60,000 years ago.

Aboroginal burning: They burnt the land for a whole range of reasons. They burnt the land so that they can move across it easily. They hunt along the fire front. They signal distant bands. Fire promotes the growth of beneficial plants that sprout after burning. So there are many reasons why humans might be burning the landscape in a way that they thought would have a positive impact. The Australian landscape is unique in that it has extremely low nutrients in the soils, because it's a very ancient, flat landscape, and most of the nutrients have already been removed from the soil. So it has a high sensitivity to burning having a big impact. Large animals in the interior of the continent were eating just such a mixture—feasting on nutritious grasses in years with good rains, and depending on trees and shrubs in drier years. After 45,000 years ago, those nutritious grasses were nearly gone, and those animals that hadn't gone extinct were restricted to dominantly tree and shrub dietary sources.

Natural Disasters: There are theories that natural disasters like a severe drought or blizzard killed the megafauna. A volcanic eruption is one of the theories too.

Mass Diseases: Diseases, infections or viruses could have swept Australia killing all the megafauna.

Change in Temperature: There could have been a big shift in temperature making it too hot or too cold for the megafauna to survive.

Invasion by non-native plants/animals. Other plants/animals are brought to Australia for certain reasons. Species that are already living there wouldn’t get used to the them. This can cause more problems.