Board Thread:Lore Discussion/@comment-24587225-20150902024621/@comment-1600847-20150903174247

It sounds like the Thalmor were planning to stop Ulfric's execution:

"As long as the  civil war  proceeds in its current indecisive fashion, we should remain hands-off.  The incident at  Helgen  is an example where an exception had to be made - obviously Ulfric's death would have dramatically increased the chance of an  Imperial  victory and thus harmed our overall position in Skyrim."