Oh, the parties are always trying to broaden their platform to appeal to as many people as possible. They want that majority number of seats, and they often get it. Very rarely do they get 50% of the popular vote, though. It last happened as part of a big swing in 1984.
But with three relatively strong parties, there's a lot of jockeying around for position. Nobody takes a really extreme position one way or the other, so you don't get quite as much polarization. If one group goes all Tea Party and extremist, they lose more than they gain and become a fringe player. I think one of the keys is that, if you're dissatisfied with a particular party but you don't want the guys who are diametrically opposed to them, there's somewhere to go.
There's more shifting around than I've seen in the US, parties popping up regionally (out west and in Quebec, for example), picking up a lot of regional support and then becoming nationally important. The 2011 election was really interesting this way, where one of the secondary parties got demolished, one of the major parties got demolished, and the perennial third place party picked up the opposition role by taking over all those seats. And in 2015, that demolished major party won a big majority.
I think a part of it comes from the fact that the leader of the party is just the leader of the party with no separate election, so if you don't like a particular leader then you vote for another party. Probably explains the swings - if Party A picks a real dud for their leader, then you vote for Party B or C. This is the biggest difference I see compared to the US. Also, because the parties are less polarized, it's easier to switch from one to another. That's definitely what happened in 2011 and 2015, the Liberal party (yes, that's the name of the party) went from having a poor leader to an inspiring one, and it's also a big reason why the usual third party did so well in 2011.
Is this part of how the system is designed? I don't think so, although it's not an unusual design or an unusual situation as far as I can tell. I am far from a scholar in this stuff, and I'm sure my historian father would love to school me on how it's changed in the last 150 years. Looking at historical elections, it looks as if it was originally a two party setup but it's evolved in the last 50 years with a third party coming in to play. Which would make for some interesting study as to how it happened and if it could happen in the US.