The trend was started in 2006 by Zero no Tsukaima.
The unexpected success of this franchise not only led to three more seasons, which as far as anime goes is very rare (even more so back then), but it also led to more isekai shows being produced.
Of course, it took about a year for the DVDs to come out and then another year for people to realize that this really was gold and not just a quick lived trend. So slowly we started to see more of this.
Isekai no Seikishi Monogatari (2009)
Dog Days (2011)
Sword Art Online (2012)
Oda Nobuna no Yabou (2012)
Ixion Saga DT (2012)
To call out a few that rode the wave Zero no Tsukaima started. By the time SAO was understood to be a very big success too, the thing was a made deal. This stuff sells, so more should be made. And the rest is history.