Former Kamen Rider Decade head writer Sho Aikawa revealed via Twitter why Kamen Rider Decade returns in next week's episode of Kamen Rider Wizard.
And I quote:
[Source: Henshin Justice]
And I quote:
It all started with a near impossible request that goes like "Can't we at least invite back one senior Rider?" but the producer claimed that "If it's not the original Kuuga or Fourze, it's meaningless." it concluded with "Or it could be Decade." and so, we put so much effort into the schedule and budget. Hence, this is what will happen next week.
[Source: Henshin Justice]