Collins Cobuild Learner's dictionary 2th (1995):
If you say that something is a case in point, you mean that it is a good example of something you have just mentioned. In many cases religious persecution is at the root of mass flights. A case in point is colonial India.
Collins Cobuild Learner's dictionary 9th (2018):
If you say that something is a case in point, you mean that it is a good example of something you have just mentioned. In many cases religious persecution is the cause of people fleeing their country. A case in point is colonial India.
As you can see, the lexicographers have changed "at the root of mass flights" into "the cause of people fleeing their country". To the best of my knowledge, a noun "flight" that means the action of fleeing is uncountable (Cambridge, Longman, Collins Cobuild said so).
But
Oxford: [uncountable, singular] the act of running away from a dangerous or difficult situation
a flight from harsh reality
Macmillan: COUNTABLE/UNCOUNTABLE the act of running away or of trying to escape from someone or something The refugees made a desperate flight to freedom.
As you can see, there's no plural of flight in those example sentences.
For this reason, I would like to see if "the root of mass flights" is ungrammatical. Plus, I'm not sure if both "the root of mass flight" and "the root of a mass flight" are possible.
"the root of mass flights" in the original sentence is correct English. However, it is relatively unusually worded, and most probably the compilers of the dictionary felt that it might be confusing to learners -- for example, that people might misunderstand it as referring to air travel. I expect that is why they changed it.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
"the root of mass flights" in the original sentence is correct English. However, it is relatively unusually worded, and most probably the compilers of the dictionary felt that it might be confusing to learners -- for example, that people might misunderstand it as referring to air travel. I expect that is why they changed it.