There was a fundamental shift in Japan's behavior in between the wars. They went from trying to be seen as a western style great power, to an east Asian power that would liberate all the oppressed east Asian nations. Since the allied powers controlled most of east Asia, Japan drifted towards the axis. Also, the late 20's witnessed the end of a period of Japanese history called "Tashio democracy", after this period military leaders became more influential and Japan became very militarized and authoritarian. This shift towards a fascist dictatorship was led by Sadao Araki. This new Japan focused on expansion and thus became enemies with the allied powers.

Sources:

Japan's Imperial Army: Its rise and fall by: Edward Drea

Edit: Taisho not Tashio.

The treatment of Japan was one of the many, many failings of the Treaty if Versailles. Japan had inflicted several defeats on Germany in the Pacific, and hoped to be regarded as equal partners at the peace conference (and to be given the islands they had taken from Germany). Instead, they were relegated to the "kids table" at the conference, and treated with the same kind of racist distain that greeted most non-Europeans/Americans at the conference. Being that it was clear that the Western Democracies would never take them seriously, Japan started to turn towards policies that contributed to a more hostile, and eventually belligerent, attitude towards the Allies.

Interestingly, this entire paragraph could easily apply to China, whose poor treatment at Versailles strengthened the Communist Party in that country, since the communists already mistrusted the colonial powers the made up the Allies.

"Change allegiance" isn't exactly the right term, they're two different wars and two different alliances even if they share members and common names.

Japan's interests and those of other states fundamentally changed between WW1 and WW2. In the first case, Japan's interests and those of others coincided, whereas around the latter, they were fundamentally misaligned.

"Allegiance"? I don't think that's the word you want; it implies that Japan was a subject, a vassal, or somehow subordinate to the western nations they fought alongside.

Like any great power, the Japanese had no friends, only interests. You might as well ask why Italy changed sides, and the answer would be the same: Interest. In the Great War Japan saw its chance to grab some German colonies on the cheap; in the Second World War it saw its chance to grab some Chinese land and Dutch colonies on the cheap. It wasn't a question of loyalty, but of what could be grabbed.