Starting from 2.7 version of SSRS we are using mechanism that determines document orientation automatically during printing.
If the content doesn’t fit the printing page we try to rotate it to fit the area.
In order to disable auto rotation add the following code:
reportViewer.UseDetectPrintOrientationFunction = false;
In this case the document will be printed on 2 or even more pages if it doesn’t fit to one page but the content will not be rotated.