You can find forgeries everywhere in the antiquities trade. Some of them are modern productions, but many (see all pieces of the true cross in churches around the world) are old enough to be historical artifacts in their own right. One of the things that makes it difficult to know if objects of uncertain provenance are legitimate is that it's very common for locals to find materials missed / never seen by archaeologists or to simply loot from excavations in-progress. The dead sea scrolls were found over decades by many different groups, so they're particularly susceptible to this. Forgers take advantage of that. Good forgeries can fool even scrupulous buyers and are mainly given away by things like incorrect radiocarbon dates or the forger's unfamiliarity with the details of the source material like the article.
If you need more context, this one article that is mostly a personality profile won't contain it. But there are hundreds of books and thousands of research papers written about the dead sea scrolls.
There are all kinds of materials used for forgeries. Ranging from cheap ones sold to tourist on contemporary papyros (or even banana leaves) made contemporary ink to highly sophisticated ones on original Ancient papyros (you either remove the old handwriting of a not so "valuable" text or use blank Ancient papyros).
> How are all of these fragments not in one place under guard?
Just an anecdote: I once visited a papyrological institute at the university I studied with a group of fellow students to receive an introduction into papyrology. All the tables in the seminar room were pushed together and on them lay a lot of papyros fragments, each fixed between two sealed panes of plastic. Among them were also the highlights of the collection: A small fragment from Qumran and a Septuaginta fragment from the 7th century. Everyone was allowed to take these sealed plastic storage frames (I don't know the professional term for it) into her or his hand and have a close look at it from both sides.