Funny thing. I'm just now writing a paper about the twist in Italy's foreign policy in 1935. In the 20's Italy's foreign policy was quite calm, even pro-western, to a degree. But from 1935 onwards, Italy got extremely aggressive and expansionist. One theory on why this might be is that Benny's internal power grew stronger and stronger, until he had absolute control over everything inside of Italy. Thus, he turned to increase his power outwards, on the global stage. France and the UK wouldn't let him upset the world's balance of power, so he turned to Germany, instead. I don't suppose you can just prevent a fascist from seeking more power, but if for whatever reason the British and French would be more than willing to let Italy rule the Mediterranean and oppress whoever the hell they want, and Italy would keep worrying about an Anschluss, than Mussolini might get really pissed if Germany does annex Austria and try to drag the whole of Europe into a war against it. Mussolini was very flexible, really, and could change policies and official position without any delays. He wouldn't mind fighting fellow fascists if it meant Italy would have its interests in Austria fulfilled.
Also another theory is that Italy's liberal values and institutions were gradually replaced by authoritarian values and institutions, until it was completely fascist and thus was ready for an aggressive, non-relenting foreign policy. I don't really know what to say about that, but I suppose grinding Italy's pre-WWI liberalism into totalitarianism is inevitable under Mussolini.