The reasons may be several, but Tim Cook has commented in an interview to ReCode that there is nothing to be alarmed about :
It may be because of the boom in mobiles with large screens that can even replace a tablet, or it may be because laptops continue to sell despite the fact that some were betting on the end of the traditional computer … if we look at all the results of the year we see that the income provided by the iPad goes from 20% to 16% . In other words: regardless of sales, they represent an increasingly smaller share of the company’s total revenue:
The future, as always happens when we try to predict something with an electronic device, can take a huge turn at any moment . All we can do is wait and take comfort in the rumours: we should see a new generation of tablets before the end of the year and their sales will tell us if the trend continues or if their sales go up again.