Revisiting the Events
Satoru sees a shadow behind him, steps away from it, and gets very close to the edge. When we return to the scene, he’s already falling down the clock tower.
We can go through each possible culprit, but most of them don’t have access to the tower. Access is blocked by Enomoto, and Enomoto himself has no motive to push Satoru off.
So how did Satoru fall? The most likely answer is that Satoru slipped off by accident as he moved away from the shadow.
But who cast the shadow?
Even if none of the other character’s climbed the tower, there is still something watching Satoru. That entity is SELF.
The chronology heavily hints SELF is related to the tower incident.
Was the "Plan" a success as expected?
Was it a failure?
Where is SELF?
The eternally inescapable maze of memory.
The shadow Satoru saw just before his fall from the clock tower——
It is already imprisoned in time-space.
It may have been "SELF"....
—— Into the Infinity Loop.
It’s not the first time a character has detected SELF’s presence. Kokoro feels the shadow in some bad ends:
I felt it. Behind me was the shadow of the existence that had beckoned us into this situation. In the evening… lit by the dawn… It had neither volume nor mass.
――A point. A hollow nothingness.
How does a zero-dimensional entity cast a shadow? It’s possible that Satoru’s paranoia fooled SELF into thinking someone actually was on the tower, so SELF unintentionally created a shadow to confirm Satoru’s fears.
If you want a more grounded explanation, we don’t need to invoke a supernatural being. The source of the shadow could be Satoru himself. The TIPS tell us about the Brocken Specter:
On a high mountain at sunrise or sunset, when one is standing in front of a bank of fog, the sunlight shining from behind you may project your shadow onto the fog, surrounded by a ring of rainbow light. This is called a Brocken specter. The resulting gigantic shadows, like those of a monster, have been called the phantoms of the Brocken. The name of the phenomenon comes from the Brocken, a mountain in Bavaria, Germany, where it has often been observed.
As Satoru moves backwards closer to the source of light, the shadow becomes larger. Before Satoru falls, he realizes the truth. The shadow he feared was created by his own paranoia and delusions. He was always running away from himself.
The chronology and the natural explanation are another hint that Satoru and SELF are one and the same.