What could be the cause of this increase? Would the trend continue? Why or why not?
I have not seen any specific rationale for the setting of the fixed rate, but my assumption would be that it's based on treasury rates to be competitive with treasury bonds (why would you buy a bond at a lower floating rate when you could get a higher fixed rate?) In short, the Treasury sets the fixed rate to be just high enough to be attractive to investors while minimizing the amount of interest they pay.
Would the trend continue? Why or why not?
No one knows - you could look at the Treasury yield curve and see if the market thinks that interest rates in general will go up or down, but even that is a guess, and does not directly affect the fixed rate for I-bonds.