Maintaining software is an ongoing process that stretches beyond the initial release. Stable software versions continuously evolve to fix bugs, add improvements, address security issues, and ensure compatibility. This ongoing support involves Backporting, which means taking a fix or update from a newer version and applying it to an older version of the same software. As software versions evolve, new technical debt can arise during backport maintenance activities. This study examines the technical debt involved in fixing 105,396 commits from 31,076 backport sources across 87 repositories in three software ecosystems (Apache, Eclipse, and Python). The goal is to identify when and why new technical debt arises during backporting in stable source code. Our results indicate that approximately 4.3% of backports introduce new technical debt. Apache contributes the most absolute instances, while Python and Eclipse exhibit nearly three times higher debt-to-commit ratios than Apache. Feature migrations make older Apache releases debt-prone in the early phase, whereas Python and Eclipse releases tend to accumulate technical debt mostly during the middle phase of their release cycles. Additionally, developers who are inexperienced, under high workloads, or non-owners are more likely to introduce technical debt during backporting.