That's not quite true JG. Tramadol, when taken in an enormous overdose (from memory the figure in the research was something like 600-1000mg), can increase the risk of seizures in about 1% of people who take it if they are already vulnerable to seizures - such as epileptics.
Somewhere down the line that always gets shortened to "it increases risk of seizure". Only in epileptics taking a massive overdose.