Hi Tink,
I STILL remember in 1962, reading a book on prejudice; speculating that somewhere in human history there had been survival value in repelling people who were "different"...I am still not sure of that, it may be more of a learned behavior?
...and I could not open your link, so went Internet prowling and found a couple more articles about Sri Lanka. Two points that stood out, one being that in Sri Lanka ethnicity and religion overlap strongly; the majority being Buddhist and Sinhalese. Easy to imagine how that might exacerbate the tensions!
Apparently also, some of the trouble might be still another legacy of colonialism (cf. the Rwandan genocide) - it seems the British gave prominence to the Tamil minority to help in suppressing the Sinhalese majority; divide and conquer.
https://thediplomat.com/2016/10/sri-lanka-separatists-or-minorities/
http://minorityrights.org/country/sri-lanka/