To understand the answer to this one, you need to have an idea of how books circulated in the ancient and medieval world. The idea of publishing houses who would have a contract with an author to copy and distribute their books (which is where we get the term "copyright"--an author assigning the right to make copies of a work to a publisher) is only about three hundred years old. Before that time, authors and poets (like most artists and musicians) generally made money only through the patronage system (although some of them, such as Cicero and Caesar, used their writings as a way of promoting their own political careers). A patron might ensure that the initial copies of an author's works were copied (or later, printed) and distributed; very famous writers might be able to arrange for remuneration (again, through the patronage system) for the first publication/distribution of works. But once a work was out there, it was essentially "public domain"; anyone could pay a bookseller to have a copy made of any work they wished, and authors would not profit. (Yes, the ancient world--and well into the modern world--worked on the precept of "exposure." Getting your work out there was a means of attracting wealthy patrons, who might commission you to, for example, write poetry for their friends or who might be able to brag about the smart people under their patronage.)

This is where the the "pseudos" come in. These titles were not assigned by the authors in their lifetimes. There was simply nothing to be gained by doing so--it's not as if pretending to be Aristotle would sell more books and earn the author more profit. Instead, these attributions were usually bestowed by later scholars, who, when encountering a manuscript that covered similar ground as a famous author, might attribute the work to that author. This would usually serve to give the work more authority. So, for example, there was a tendency to assign authorship of any work that covered, say, geometry to Euclid. This was particularly prevalent in the Middle Ages, where the transmission history of the works of many ancient authors had been lost.

It wasn't until scholars began more systematically studying and comparing the works of various ancient and medieval authors--starting during the Renaissance, but not really picking up speed until the modern era--that various works began to be identified as "pseudo". . Modern scholars can analyze and posit whether a text could be the authentic work of a particular author through analyzing known works by an author to whom a text is attributed for both how they use language and how the ideas compare with those presented in works known to be authentic. Sometimes the language itself (for instance, use of Latin forms and grammar from a couple of centuries later in a work attributed to Cicero) can be an important indicator. It's important to note that "pseudo" sources are often vitally important sources when used critically--they're just not the work of the author to whom they were originally attributed

So to sum up: it's not insecure ancient authors who are responsible for "pseudo" works. It's later scholars who made the initial attribution to a famous author based on its subject matter and to augment its authority, and then still later scholars who were able to identify that, for whatever reason, a particular work had been incorrectly attributed.