Precisely this, yes.

Anecdotal: I have had many exposures to the illicit drug markets in my areas throughout the years. The symptom of many underlying unresolved issues I am sure, but that is not relevant. What I wish to say is that I have seen many pressed counterfeit tablets in my lifetime and have had the luxury and wherewithal to test a majority of them before usage. I have personally never identified fentanyl in a product that was not marketed as an opioid. With illicitly manufactured oxycontin and its ilk you will come across it a majority of the time. But with other things such as MDMA, ketamine and such it's incredibly rare to see. That is not to say it doesn't happen however, there have been instances of things containing fentanyl which had no business doing so. Despite this I echo GP's claim that it's largely due to inadvertant cross contamination than it is some intentional reason. The supply chains for these drugs are very long and windy once they are finalized for consumer use inside the US as well, with many middlemen making a few dollars to facilitate a transaction. Each of those hand-offs is one more opportunity for the new seller to say "I bought X, but could sell much more if I said it was Y", and that ends up killing people occasionally too.

Hypothesis: There is a number of factors at play here that add to the death toll. There is the occasional unintended cross contamination during production. It's also worth noting that individuals who import bulk drugs from overseas for their own sales often order multiple different substances and craft them for sale, further increasing the chance for that contamination during production. There is the fact that fentanyl and a majority of it's analogues are active at shockingly low doses relative to more classical opioids, which makes improper distribution of active ingredients into the final pill mixture highly likely when you consider they are usually not created with the best quality control procedures or equipment. There is the fact that younger users (I would argue they are more experimenters than users per se) of opioid drugs have less physical tolerance to these chemicals. We should also consider that youths also have less practical experience, and not knowing the "lay of the land" with these substances can kill you very easily. And then of course the obvious increase in the average American's listlissness, hopelessness, economic uncertainty, fear of the future and so on. The ground under our feet is shifting and drugs are a temporary respite from that, if harmful long term.

Add to this the fact that we have a very drugs-illiberal society on the whole, whose governments oftentimes think the best way to address this problem is simply banning substances or once in a while executing operations to damage the supply chain, while completely avoiding the idea that perhaps most usage of these substances is symptomatic of a problem and not the problem itself? And you end up with an firehose of drug analogues that can escape easy regulation and restriction juuuuust long enough to gain a foothold in the market and shift the consumer culture of a particular drug. It's not a problem you can legislate your way out of, and it never will be.

All of this is to say I expect US overdoses per capita, the "never-before-seenness" (and by extension lack of medical knowledge surrounding it) of the drug analogues on the market, and the overall appetite for drugs of dependency, all to continue increasing for the forseeable future. I wish it wasn't this way. But like most of the US' modern cultural issues we enjoy arguing about what could offer relief far more than we do finding a bipartisan way to address them.