Why did India originally support the LTTE in the Sri Lankan civil war and then oppose them?

From my reading, India's RAW trained Tamil separatist fighters starting in 1983 and in 1987, supplied LTTE forces during Operation Poomalai. However after the Indo-Sri Lanka Accord, the Indian Peace Keeping Force started aggressively going after the LTTE. What changed to make India attack its former ally? This was before the Rajiv Gandhi assassination so that couldn't have been it.