The main culprit is myrcene, American hops are often very rich in myrcene, and it accounts (primarily) for flavours described as Soapy or Cheesy, there are also a bunch of Terpens and Keytones as well as some minor hop oils that we are best rid of. There is also a chemical path way that involves insoluble hop fractions becoming Alcoholised (gaining an OH) and becoming soluble, this takes some time, once they have gone into solution they can be scrubbed from the wort with the steam, examples would be Linalool, Geranoil and Humulerol among others.
Myrcene isn't all bad; in fact it can be really nice, if you use it either early or late, in my experience most of the unpleasant tastes in (particularly) American beers can be traced to midpoint hop additions.
MHB
This is what Brewing Wiki has to say.
Myrcene yields flavors that were not traditionally considered desirable by European brewers, and noble hops are very low in myrcene. However, many American hop varieties are very high in myrcene; it makes up up to 60% of total oil in Cascade and up to 70% in Amarillo. Also found in some citrus fruits, myrcene lends American hops many of their distinctive flavors.
When added late in, or after, the boil, myrcene adds the intense, pungent aroma associated with American dry-hopped beers. When boiled for longer periods, it yields the characteristic citrus and pine aromas of American craft beer.
