How souls fall from the celestial planets with the rain?

How it can be that a pious soul coming down from Svargaloka is immediately put into the body of a plant? Is the process of being eaten and becoming a baby just accidental?

It's mentioned in the Srimad Bhagavatam that when souls fall from the celestial planets, their good karma exhausted, they fall down with the rain, become plants that grow grains, and being eaten by a man eventually become semen, which is then transferred to the womb of a woman, leading to a new birth as a human being:

"My dear King Yudhiṣṭhira, when oblations of ghee and food grains like barley and sesame are offered in sacrifice, they turn into celestial smoke, which carries one to successively higher planetary systems like the kingdoms of Dhumā, Rātri, Kṛṣṇapakṣa, Dakṣiṇam and ultimately the moon. Then, however, the performers of sacrifice descend again to earth to become herbs, creepers, vegetables and food grains. These are eaten by different living entities and turned to semen, which is injected into female bodies. Thus one takes birth again and again." (SB 7.15.50-51)

The same is confirmed by Prabhupada in his purport: "After one's enjoyment due to pious activities is finished, one must return to this planet in rainfall and first take birth as a plant or creeper, which is eaten by various animals, including human beings, and turned to semen. This semen is injected into the female body, and thus the living entity takes birth. Those who return to earth in this way take birth especially in higher families like those of brāhmaṇas."

The main point of this verse is that any position in the celestial planets is temporary since when one's pious merits are exhausted, the soul will inevitably fall from the celestial planets back to Earth, where one will have to again follow the path of religious life and fruitive sacrifices if the idea is to ever be promoted again. In this way, the scriptures condemn this path of temporary elevation to Svargaloka.

However, one could question this description of the soul falling with the rain and becoming a plant. How it can be that a pious soul coming down from Svargaloka is immediately put into the body of a plant? Is the process of being eaten and becoming a baby just accidental? Is the grain with the soul immediately eaten, under a superior arrangement, or the soul has to wait until a chance appears? What happens if instead of being eaten by a man the grain with the soul is eaten by an animal or insect?

In his comment to the Vedanta Sutra (section 3), Srila Baladeva Vidyābhūṣaṇa describes this process in more detail and dispels many of these doubts.

In the celestial planets, the jīva possesses a subtle body. When the soul falls from there due to the exhaustion of his pious karma, the soul goes through a process of developing again a gross body on the Earth. First the soul associates with the element ether, and then air, fire, and water.

Associated with the element water, the jīva falls into the earth as rain and associates with a plant that produces grains. Just as the soul does not become ether or water, but just uses these elements as a transport, the soul also does not become a plant but just uses the plant and the grains as a medium to be transported into the body of a man. There, again the soul does not become semen but just uses it as a transport to enter into the body of a mother where he can develop his human body and finally take birth. This process is not accidental: everything happens due to karma.

The plant, the grains, and the spermatozoa in the semen have all their corresponding souls who are enjoying these particular bodies, however, none of them become human beings. The one who gets the human body is the soul which uses all these elements as a medium and is predestined to get a human body.

It's amazing how everything happens in an orderly way in nature. It's only due to our ignorance of the true causes that we think things happen by chance.

As Srila Baladeva Vidyābhūṣaṇa comments: "The jīvas only contact the plants, since other jīvas inhabit these bodies as enjoyers. These jīvas do not appear there for enjoying those plant bodies. Why? Because of what was previously stated (pūrvavat): just as the jīvas become like ether, they become like plants. When the jīvas pass through ether until rain, it is not described as karma for enjoying there. Similarly when they enter plant life it is not described as karma for enjoyment. When such enjoyment is permitted, it is described (abhilāpāt) with words like "pleasant karmas (ramaṇīya-caraṇā)." Therefore the jīva simply contacts the plants but is not born as those plants." (VS 3.1.25)