what right would anybody from dorne have to go at the lannisters though....oberyn volunteered to be in the trial by combat....he got killed fair and square. nobody to blame but himself.
Its like this: he saw and opportunity to take revenge outside of the usual diplomatic channels but taste of revenge was too sweet...too perfect
The night before he was probably thinking about all the moves and words he was going to do like a child having his first fight at school.