To bring up the old Cold War phrasing...
Why would Thailand suddenly jump ship arms-wise and sign a huge Sukhoi deal?
There's only one thing that makes any sense to me, and that is that with Thailand recently sitting unreservedly in the pro-China camp of ASEAN, that they want to obtain weapons systems that allow for interoperability with their northern neighbor.
But that's probably a bit of a stretch. Anybody got any good ideas?