I was in the country during this time and I remember it being more like a rival faction arresting their political opponent. I first came to Thailand in 2005 so I missed what happened before this point but It's all very similar to what's happening now to Trump in the US. Basically a non-establishment individual enters the government, despite their best efforts to keep him out, and once they're in the knives are out and they do what ever they can to expel them.
I bet they attempted various things before finding this corruption charge that stuck, just like the Russia collusion story that landed on Trump immediately after he took office.