Is it normal for PIs to change projects to unrelated research
I would say, no, it is not normal for a PI to shift focus so radically.

In the normal run of things, when hiring at the PostDoctoral level (and lower), the PI will have: 


Submitted a funding request with a detailed outline of the research plan.
Have received acceptance of that plan and a budget.
Advertised open positions accordingly to the budget looking for people with the necessary skills.
Hire people with the necessary skills for the duration/budget allowed by the funds.
Supervise execution of the research according to the initial plan.


... pretty much in that order.

Topic shifts would normally occur where funding for a project has run out (which should correspond with the end of the PostDoc contract) and the PI wishes to find funds from other projects to keep the PostDoc employed and to extend the contract. 

Shifts might also happen if the PI is pooling sources of funding together, which can sometimes happen: they might decide to juggle resources between different projects on the fly. This, however, is far from an ideal situation, esp. for managing researchers on temporary contracts.

The questions I would ask in your situation are: in which of the PI's projects you are working on, has it changed recently, where the funding for that project came from, and what the original proposal was. 


If you have moved project, you should ask why. 
If you are in the same project, you should ask why the topic changed so radically all of a sudden.