In short...
During 1880's Sri Lankan tea became highly in demand over Chinese tea, so English had to produce it to match the demand, and Sinhalese were utterly disgusted about English so majority of them would not work under English.. (ofcourse who would work under someone whom has your land by force

) so they imported Tamils from India (there were small percentage of Tamils in SL before that too) and English started using them as the work force, and they (Tamils) also helped English in their administrative work!
English were impressed about them... gave them privileges that Sinhalese never had like Education, Health etc. and when Sinhalese ask for the same they said that it was Indian govt's wish!
Furthermore, English used Tamils to spy on Sinhalese rebel leaders and there was a council which English appointed 1 Tamil to represent Tamil communities and none for Sinhalese.. as the time goes by India was struggling for its independence and so does Sri Lanka amid the high pressure of overwhelming Sinhalese majority English had to include Sinhalese in the council - And then Tamils felt threatened because now Sinhalese and Tamils were one to one and after some time English did raise the number of Sinhalese councilmen to 13 acccording to the population ratios.. which suggests that English knew this land had at least 80% of Sinhalese at that time
Tamils did not like this a bit.... after all those ASS LICKING all they got was 1 council men... but by now they knew that English could no longer hold on to Sri Lanka as India have already gotton thier independence so they slowly but cunningly came over to join Sinhalese independence movement, yet their mentality of a 50-50 representation did not go anywhere... after the independence they've asked for 50-50 representation but Sinhalese would not entertain such unfair rules, and they've had enough of Tamils and their treachery and Sinahlese were surpressed for a very very long time by many colonization finally helped by Tamils too...
So came the 1983 incedent many Tamils got killed, and then Praba was born to start his movement..
I personally feel that 1983 indecent was very stupid of Sinhalese to do such a thing.. but it was on the cards for a very very long time... and today Tamils all around the world try to present this indecent as if Sinhalese wake up one morning and start to kill Tamils as if they are cold blooded lunatic killers, which is totally not true and a very unfair comment, yes Sinhalese killed Tamils in 1983 but the reason fuel that event was Tamils themselves and their greedy politicians asking for stuff that are totally undue.