You go out away from you too much to start. Thus, what it "causes" is for you to then go too much "around" you the rest of the backswing. This puts you in a "too shallow" position (notice where the shaft is pointing) and requires you to steepen it a precise amount exactly at the moment when you have a tremendous amount of force and very little time to act on the club (transition into the downswing).
Weight WAY back in your heels.
Hands go out away from you.
Hands track around, club is super shallow and not pointing anywhere near the target line.
Club MUST steepen in transition to get anywhere close to the ball. Bad time to do this.
The driver swing is much worse, as at least with the irons you maintain something of a neutral shoulder tilt. Not so with the driver:
Big trouble there.
See this topic for your setup:
You ain't Rickie.
You're miles away from being able to "feel on the way down." You need to spend a lot of time on the first foot of your backswing, and then the first three feet, and then the next three feet…
After you spend time on your setup.
This is a situation in which the Committee could have (should have?) defined the downed tree as GUR, in which case you would have been able to take free relief. In my group of friends, we'd probably play it as GUR in a casual game. Otherwise, a completely detached fallen tree is a loose impediment, you could have moved it (if possible) or even broken off part of it in order to make a swing. If its still attached to the roots, you simply cannot do anything about it.
But a question, in taking Unplayable Ball relief, couldn't you have gone back on the line and found a place where you could swing? You don't have to take Lateral Relief and stay within 2 clublengths.
Last year some serious wind took down a tree and it was laying horizontally. Played that course a lot, for golf reasons, that day was the first time I ever hit one anywhere near that tree. Because of course it was. It was under the fallen trunk/branches so much that even after taking an unplayable and relief I had no swing. It felt really dirty but the guy I was playing with said the fallen tree didn't have any option for free relief and since I had not realized it was under the tree from the box, and did not want to do the walk of shame on the first tee with multiple groups waiting...ugh. Sometimes the rules feel penal, but then I remember I am the one who made the sub-optimal swing in the first place.