Near the end of "Do Androids Dream of Electric Sheep?", someone throws a stone at Deckard the way they were throwing stones at Mercer as he was climbing up a hill. Is it ever explained or implied who throws the stone? Mercer wasn't really in the real world, in a way, but Deckard was; there must be SOME explanation given for this. Rocks don't just randomly throw themselves at people, after all. Please note that I am looking for an in-universe answer as opposed to the metaphorical meaning or whatnot.
No one (probably)
He walked on, up the hillside, and with each step the weight on him grew. Too tired, he thought, to climb. Stopping, he wiped stinging sweat from his eyes, salt tears produced by his skin, his whole aching body. Then, angry at himself, he spat - spat with wrath and contempt, for himself, with utter hate, onto the barren ground. Thereupon he resumed his trudge up the slope, the lonely and unfamiliar terrain, remote from everything; nothing lived here except himself.
The heat. It had become hot, now; evidently time had passed. And he felt hunger. He had not eaten for god knew how long. The hunger and heat combined, a poisonous taste resembling defeat; yes, he thought, that's what it is: I've been defeated in some obscure way. By having killed the androids? By Rachael's murder of my goat? He did not know, but as he plodded along a vague and almost hallucinatory pall hazed over his mind; he found himself at one point, with no notion of how it could be, a step from an almost certainly fatal cliffside fall - falling humiliatingly and helplessly, he thought; on and on, with no one even to witness it. Here there existed no one to record his or anyone else's degradation, and any courage or pride which might manifest itself here at the end would go unmarked - the dead stones, the dust-stricken weeds dry and dying, perceived nothing, recollected nothing, about him or themselves.
At that moment the first rock - and it was not rubber or soft foam plastic - struck him in the inguinal region. And the pain, the first knowledge of absolute isolation and suffering, touched him throughout in its undisguised actual form.
The inguinal region refers to the lower abdomen.
Note that the hillside is covered in loose rocks.
A cluttered hillside swooped up at him; he lifted the hovercar as the world came close. Fatigue, he thought; I shouldn't be driving still. He clicked off the ignition, glided for an interval, and then set the hovercar down. It tumbled and bounced across the hillside, scattering rocks; headed upward, it came at last to a grinding, skittering stop.
Deckard is climbing up a steep hillside covered with large, unstably situated rocks and pebbles. As he walks up the hillside, he must be displacing rocks. This will cause the rocks higher up, which were resting on them, to tumble. If they bounce a bit, they could easily hit him in the groin area or lower abdomen, on a sufficiently steep him.
Further, if someone had thrown it at him, he would have seen it. It was coming from above and in front of him, after all.
Who threw the stone at me? he asked himself. No one.
While it is a thematic callback to Mercer's experience, the situation is different.