That backprop is most likely an artifact of a sample on a lineage being sequenced in North America, but the lineage also continues in Asia, where that lineage had not been sequenced yet. As in, someone in Asia spread COVID-19 to two people, one of whom traveled to North American and had their virus sequenced. The lineage also continued in Asia and was eventually sequenced days/weeks later, so the inferred tree includes a North American sample seeming to give rise to more Asian infections.