To reproduce:
1. Open attached project "AssetBundles 4.3"
2. Open scene test. It contains a button that when pressed will try to load 5 asset bundles and unload each one after it's been loaded
3. Enter play mode and press the button
4. Notice that after the first assetbundle has been loaded and unloaded unity freezes until all other assetbundles are loaded as well (you can see this via console)

It should not freeze and continue loading as usual