"Major genera" is too broad a set, I think, to answer that one accurately, since some species and hybrids within a genus will exhibit that behavior, and some won't. A grower's conditions will play a big part in that as well. That said, monopodials like Phals and Vandas should never drop all their leaves. If they do, something is definitely wrong culturally, though the loss of a lower leaf here and there on mature plants is perfectly normal.

A sympodial like Cycnodes, for instance, will lose all of its leaves for its winter rest before putting out a new growth, provided the grower allows the temperatures to fall low enough. If the temps aren't dropped, the plants will carry their leaves into the begining of the next season, well after new growth has already started.

Many nobile dendrobiums can also lose all the leaves on an older cane just before budding, and if you don't expect that, it's easy to think that your normally slow-growing plant is suddenly dying in front of your eyes in a matter of days. However, many within the genus Dendrobium won't do that, especially the ones that don't especially require a cold winter rest to initiate buds.

A host of Cypripediums and other genera native to the US will die back to "roots and rhizomes" come winter, leaving virtually nothing left alive above ground.

To my knowledge though, all orchids go through some kind of rest period after putting out new growth and / or blooming, a time when they seem hung in suspended animation and appear to be doing absolutely nothing. During that time, they might drop leaves or even allow entire growths to die back in preparation for another growth spurt, either next growing season, or, in the case of some hybrids that can rebloom a couple times in one season (some in the Catt alliance, some Oncidium intergenerics), next month. The usual practice is to water just enough to keep any pseudobulbs from shriveling, and only resume normal watering and fertilizing when the plant itself has given you the go-ahead by putting out new roots, growths, or buds.

I'm sure others with experience growing specific genera can elaborate more on this, since very few of the sympodials I grow here die back to bare bulbs on a seasonal cycle.

Without knowing what kind of plant Helen is growing, there's really no way to answer whether what's happening is normal for that type, or whether she's doing something wrong culturally. For example, if it's a Phal dropping all of its leaves, or a Dendrobium dropping all of its leaves on new growth, my first guess would be an overwatering / root rot or underwatering problem. No one can say though, without having more info.

(BTW, I can't pipe. But I can sure blow pretty good. )