The closest I can think of is human-induced seismic activity, due to fracking or wastewater injection, for example. However, I do not know if this could conceivably also affect in any way nearby volcanism. I also don't think there are or have been any oil-related activities around Mount St. Helens, but I really don't know for sure. So, is this possible?
On the other hand there is passive interaction in that many terrestrial volcanoes in wet climates produce vast quantities of volcanic ash that weathers rapidly to produce fertile soil. This fertility attracts farming communities who put themselves in harms way, being at risk from hot ash clouds, hot lava, volcanic bombs and mudflows. The latter are produced by hurricane level rainfall mobilizing the loose ash, such that mudflows (or 'lahars') kill far more people than the volcanoes themselves - we have seen many examples in the news in recent decades - in South and Central America, Indonesia, Philippines, etc.
The Mount St. Helens eruption was caused by the Juan de Fuca tectonic plate subducting under North America. It's a 100% natural process, with no human causation whatever.
There are no known cases of fracking having any impact upon volcanoes. Can you imagine any fracking company taking the legal risk of setting off a volcanic eruption? In any case, the basic geology of volcanoes and hydrocarbon bearing sediments is quite different.
Kutterolf et al. (2012) showed recently that climate affects the frequency of volcanic eruptions, chiefly through changes in global sea-level and isostatic effects. Ice unloading can also trigger eruptions in places like Iceland, as researched by Sigmundsson et al. (2010). So it seems highly likely that human-caused climate change will affect the frequency of volcanic eruptions, although according to one of the scientists:
We predict there's a time lag of about 2,500 years," [co-author] Jegen said. "So even if we change the climate, you wouldn't really expect anything to happen in the next few thousand years."
The lead author adds:
The impact from man-made warming is still unclear based on our current understanding.
Clearly this climate cause is rather indirect. And certainly the extent to which it's an issue today is open to debate. As you suggest, there may be some others causes. But, either way...
Humans can contribute to disasters in other ways than being the root cause of the natural phenomenon. They are relevant to how natural phenomena affect humans; if they don't affect humans, we don't call them 'disasters'. Think about things like the following:
Read about hazard evaluation, monitoring, and avoidance, for example around famous recent eruptions. Some of these, such as Montserrat, are especially well studied. This BGS page has some more info on that volcano.
Humans almost certainly had something to do with a mud volcano eruption, which displaced thousands of people in Indonesia in 2006: the Lumpur Lapindo eruption, which is coming up on its 10th anniversary. Read my article from it's 8th birthday, and see this question. Obviously, this is a completely different kind of volcano.
If you want to start an eruption, Erik Klemetti (a legit vulcanologist) has some tips.
Reference
Kutterolf, S., M. Jegen, J. X. Mitrovica, T. Kwasnitschka, A. Freundt, P. J. Huybers (2012): A detection of Milankovitch frequencies in global volcanic activity. Geology, G33419.1, DOI 10.1130/G33419.1. Read more.
Sigmundsson, Freysteinn, Virginie Pinel, Björn Lund, Fabien Albino, Carolina Pagli, Halldór Geirsson, Erik Sturkell (2010). Climate effects on volcanism: influence on magmatic systems of loading and unloading from ice mass variations, with examples from Iceland. Phil. Trans. R. Soc. A 2010 368 2519-2534; DOI: 10.1098/rsta.2010.0042.