I have seen this problem over and over and over - It is caused by MOD scripts conflicting with vanilla game scripts during the Helgen sequence - The entire Helgen sequence until you exit the cave is an intro and tutorial. It is heavily scripted and jumps back and forth between movie type scenes and action sequences. You actually have almost no real choices during the action sequences - all you can do is follow the script. - it is handled a bit different that the real game that actually starts after you exit the cave. Nearly all of the Helgen into/tutorial scripts end when you exit the cave. Very few mods do anything at all during that sequence so if they are not doing anything - why not just disable them to prevent these wacky problems? It is those that do try to do things that conflict anyway. Mods that don't run scripts don't cause problems. - so you can pretty up your PC if you want. Although custom races may cause some problems.
This is an overreaction, tbh. The only mod I've ever had problems with during any of the Helgen sequence(s) is Realistic Ragdolls and Force, which threw Lokir from the cart before the title card faded and sent all the carts rolling down the road in front of their horses. And even then it was simply a conflict with another mod, I'm sure, because I've used it since without such issues.