The Visigoths crossed the Danube into the Roman Empire in 376. They were allowed by the Romans to settled in Thrace in 380 and became
foederati, like the Franks in Belgium. Under Theodosius, the Visigoths sacked and settled around Constantinople. Under his successor, Alaric I, ravaged Greece (as far as the Peloponese) in 396 before being driven back to the mountains of northern Greece and Macedonia. Whether some of them stayed behind, or whether they raped local women is unknown, but I2b and I1 had to get there one way or another.
I believe that one reasonable explanation is that the descendants of the Visigoths who had settled in Thrace dispersed to Macedonia, Greece and North-West Anatolia progressively over time. After all, they had become citizens of the Eastern Roman Empire, so why should they stay in one place when they could seek their fortunes further away within the empire's borders ? Just by marrying/moving from one village/town to the next each generation, the lineages would have dispersed naturally over the centuries. But if that is what happened, we should still expect a higher percentage in Thrace, shouldn't we ? Not necessarily. The Bulgars invaded the region a century later, and are known to have fought the Gothic armies of foederati. It's possible that many Gothic families, sentimentally unattached the the land of Thrace, sought refuge into the relative safety of the mountains of Epirus and Macedonia around that time.
With a bit of imagination we could come up with several other scenarios. The thing is that countless events went unrecorded in history, and others that were recorded at lost today, especially those from the Antiquity. So we will never know what or how it happened, but the point of population genetics is justly to help us better understand history by looking at what people left behind them, their genes, and try to rediscover what history has forgotten.