I have a suggestion for a partial answer.
The mandragora may have gradually evolved to absorb some forms of nutrition via its skin, in contact with the soil. Some creatures form symbiotic relations with algae and other creatures that live on/in their surface layers and produce nutrition from the environment or light, and our own mitochondria started as separate forms (I think), so perhaps this is plausible.
Over time the soil being rich they lost much of their mobility, and became a creature that is largely static and inert, and only slowly moves place to place. Mentally, if they were intelligent before, then they became introvert philosophers, and gradually thought and looked outwardly, less. If they were sentient but not highly intelligent perhaps they regressed.
(This would make sense; intelligence no less than other traits, is subject to evolutionary change, it takes a lot of energy and has no special privilege genetically speaking beyond any other survival/reproduction factor, so it may well be lost if the advantages are few and other changes to behaviour, environment, or other matters affecting their species push them in different directions).
As their food/nutritional sources became gradually more co-dependent with their symbiotic partners, they developed defences for the photosynthesising symbiotic creatures on what was their heads, and chemical defences in their dermal.layers below ground (slightly but not excessively rooty, and also home to soil converting bacteria/symbiots). In air, acoustic was less costly in evolution terms for reasons of past biology.
(Symbiosis often provides defence/favourable environment benefits to one of the partners and food for the other, so this is quite common and would be a sensible and even likely development.)
They only move slowly, but they live a long time. They give birth by partuition or external placenta equivalent (maybe that was more common in their ancestral world or life form and wasn't unusual), and as with humans, the young need a favourable environment and nutrients; they form underground. Chemicals given off by their symbiots deter the few underground predators such as moles or rodents that could pose a risk, until the new mandragora splits off and begins its own lifelong and very slow travel. This wouldn't be a difficult adaptation; even among real-world creatures many/most have neonatal stages inside bodies or below ground level. The new mandragorae have all their early needs met from parental nutrient (and warmth if needed, although cold blooded might fit better), and symbiots from their parental dermis quickly colonise their own dermis. Their banshee scream is for multiple reasons:
- to defend themselves
- to defend the was-head-based symbiots, and especially to defend the soil/below ground symbiots, which will die if exposed/dried, and which hurts them too. (The ancestral symbiots ensured they were cared for by dumping pain-causing chemicals into the mandragora if suffering; over time the mandragora evolved to take the symbiots' pain as its own.)
- because if it takes 20 years to move 1/4 mile to a nearby heterogynous mandragora, you'd scream too if someone tried to get in your way.......