Much better graph from the source that illustrates where the cause lies:

"Synthetic opioids other than methadone" is mostly fentanyl. I highly doubt that such huge numbers of people are having lethal overdoses of tramadol and the like. Also shows that numbers from other opioids have remained mostly stable throughout, in comparison. A small but significant bump in heroin, that subsequently decreased. Could be linked to poor detection of fentanyl laced heroin that has improved over the years.