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Why do several isoglosses sometimes form a bundle along the same boundary?

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Thematic map of Europe displaying the distribution of Germanic languages, with different colors indicating regions where English, German, Netherlandic (Dutch), Frisian, Swedish, Danish, Norwegian, Icelandic, and Faroese are spoken. Striped areas mark regions where bilingualism is common, and language regions cover countries such as the United Kingdom, Germany, the Netherlands, Denmark, Sweden, Norway, Iceland, and parts of surrounding countries. — Dialect Distribution of the Germanic languages in Europe.

In linguistics, an isogloss represents the boundary line for a specific linguistic feature. While individual isoglosses often cross and interweave, they sometimes cluster together to form a bundle.1

Several isoglosses may form a bundle along the same boundary due to three primary factors:

Geographic obstacles: Physical barriers can impede the diffusion of multiple linguistic innovations, causing them to stop along the same line.1
Historical circumstances: Long-standing political borders often serve as barriers that prevent the spread of linguistic features, leading to a concentration of differences along those lines.1
Migrations: The movement of populations can bring two groups into contact whose dialects developed in noncontiguous areas, resulting in a sharp transition where multiple linguistic features differ simultaneously.1

These bundles are significant because they often allow linguists to divide a territory into distinct regional dialects, marking a clear transition in the linguistic landscape.1