In below sentence, why verb "trigger" need to be singuler and not plural as "Consumption" is treated as the subject of the sentence?
Consumption of bread products made from ergot-infected grains often trigger severely debilitating symptoms such as muscle contractions, seizures and gangrene but, at a proper dose, also cause a significant reduction in maternal bleeding after childbirth.

But in below sentence:-

A higher interest rate is only one of the factors, albeit an important one, that keeps the housing market from spiraling out of control, like it did earlier in the decade.

Why "Factors" is considered as subject and verb "Keeps" need to be plural and why not we are considering "A higher interest rate" as a subject and then treating verb accordingly? What is the best strategy to follow to determine subject and then correct verb tense in such situations?

In your first sentence, you have:
Consumption [prepositional phrase (of bread products)] [modifier of noun in prepositional phrase (made from ergot-infected grains)] triggers...

Consumption is singular and trigger is singular.

In your second sentence, you have:
A [adjective (higher interest)] [initial subject and verb] rate is [adverb (only)] one [prepositional phrase (of the factors)] [aside (albeit an important one)] that [modifier of most closely preceding primary noun (keep the housing market...)]

Core is: A rate is one of the factors that keep. The "that" is a noun modifier, indicating that it is referring to the closest preceding primary noun. "albeit an important one" is just an aside - it can be stripped from the sentence completely without losing any meaning. So the closest preceding primary noun is "factors" - hence "factors that keep"

The rate, by the way, already has its verb: is.

Thanks Stacey. Good explanation

No problem! :)