Why is a manuscript tradition called contaminated when it contains readings from multiple sources?

In the context of textual criticism, a manuscript tradition is called contaminated (or "open") when a copyist has utilized more than one exemplar or incorporated variants from multiple sources into a single witness.1

This process is referred to as "horizontal" transmission, which contrasts with "vertical" transmission—a "closed" tradition where a copyist follows only one model. In a closed tradition, scholars can use the genealogical or stemmatic method to construct a family tree (stemma codicum) of witnesses to reconstruct the original text. However, when contamination occurs, the relationships between manuscripts become significantly more complex, often making the construction of a clear stemma impossible because multiple textual states coexist within a single document.1

Essentially, contamination represents a blending of different lines of tradition, which complicates the critic's ability to trace the lineage of a text back to its archetype.21