Well, I’m not sure you would count Germany, but here’s what happened after WWII (heavily abridged):
After the experience of the multi-party system that was the Weimar Republic and what it led to, German founding fathers instituted the 5% hurdle - you can only be voted into national parliament if you have more than 5% of the national popular vote. The same rule applies respectively to statewide and communal elections. This keeps out fringe single-issue parties while allowing smaller parties with a broader program. From the beginning of the Republic, there were two major parties, SPD (social-democrat) and CDU (“Christian”, conservative), and one small party, the FDP (“Liberals”, although that word does not mean here what it means in the US - think socially liberal, fiscally conservative, individual freedom, low taxes). Never in the history of Germany was one of the major parties able to govern on their own, so there were always “coalitions” formed, usually one of SPD and CDU together with the FDP. There was a brief stint of a so-called “great coalition” of the two major parties that pretty much resulted in standstill, so it is not generally regarded as a good idea.
From the environmental movement of the 80s, the Green party evolved. They quickly gained momentum and were soon elected into state and national parliaments, as the peace and anti-nuclear movement was very strong at that time. After the “Wende”, the former socialist party of the GDR managed to transform into an at least superficially democratic party and has been around ever since in various permutations. The last incarnation is called “The Left” and has managed to garner support in both the former East and West. Three years ago, as all over Europe, the Pirate party was formed in response to the lack of answers to online-based problems from established parties. They have been elected into many state parliaments and stand a pretty good chance in next year’s national election. God help us.
So the German party system has, over the course of 30 years, moved from a 3 party system to a 6 (or more) party system, and as you can see, there is not one reason, but each party had their own time to develop and form their base. These bases and ideals have sometimes shifted over the years, for example the pacifistic Green party had to swallow the fact that their own Joschka Fischer, secretary of state from 1998 to 2005, played a big part in the NATO intervention in Kosovo, and nowadays, the Green party is less of a single-issue tree hugger, granola bunch, but rather the third biggest political force in Germany, with a broad range of policies.