As depicted below the Bulgars seem to have originated just north of the Crimean peninsula. http://upload.wikimedia.org/wikipedia/commons/d/d5/Pontic_steppe_region_around_650_AD.png If this is where they came from then how did they end up in current day Bulgaria? Was this due to the first Bulgarian empire? If so, why did they leave their homeland?
Besides that how did some end up as the Volga Bulgarians?
As a last side question; on this map we also see the Goths in the Crimea. Why is that? Aren't the Goths originally from Poland/Eastern Germany?
To answer your question, the standard narrative that all the Bulgarians, including both those both would establish the First Bulgarian Empire and those who would become the Volga Bulgarians came from an original homeland called Old Great Bulgaria.
The Byzantine Patriarch Nikephoros I said that the reason the Bulgarians left Old Great Bulgaria stemmed from the death of the Bulgar king Kubrat and the fact that he had five sons. While the five sons had been told to remain united, they instead took people loyal to them and went their separate ways, as often happens with nomadic groups. The eldest, Baian or Bayan stayed in the Bulgarian homeland, which became greatly weakened by the divide and was ultimately conquered and absorbed by the rising Khazars ( a nomadic group famous for later converting to Judaism). The second son, Kotrag, took his followers up to the Volga and Kama rivers and it is from them that the Volga Bulgarians are said to stem from. The third son, Asparkuh, took his followers and settled near the Danube, and it is this group that would later give rise to the First and Second Bulgarian Empires. the Fourth son, Kuber attempted to seize control over the Avars but failed and ended up settling the Pannoina. The Fifth son, Alcek, ended up in the Italian Peninsula paying tribute to the Byzantine (Roman) Empire. This graph shows all the Bulgarian migrations: http://en.wikipedia.org/wiki/File:Bulgarians_and_Slavs_VI-VII_century.png and here are two articles on Great Old Bulgaria and the Bulgarian migrations:
http://www.kroraina.com/p_bulgar/p_bulg5.htm#n.17
Of course, we cannot be certain if this is exactly how or why the Bulgarians migrations occurred, but they do seem to fit into how nomadic groups get built up and then dissolve. The classic examples here being Attila uniting the Huns under his leadership, who then essentially disappear after his death or how Genghis Khan united the various Mongol tribes into a unified and very effective fighting force.
As for the Goths, while the Gothic legends say that they came from Scandinavia (Gotland in particular), the first written sources about them are as a people living on the Danube river in what is today the Balkans. While most of the Goths eventually went into the Roman Empire (Visigoths and Ostrogoths), there was a contingent of Goths who migrated into the Crimea and conquered the Hellenistic Bosporan Kingdom. These Goths lasted until the 18th century, when Crimean Gothic finally became a dead language. For more information on the Crimean Goths, I may advise to take a look at http://rbedrosian.com/Byz/Vasiliev_1936_Goths.pdf. It is probably a bit outdated but it should at least provide a good intro.
TL;DR: Both the Danubian Bulgarians (First Bulgarian Empire) and the Volga Bulgarians came from the same homeland on the Black Sea. They broke off after the death of Kubrat with his five sons dividing the tribe among-st themselves and travelling off.
The Crimean Goths were the third, the smallest and the longest lasting branch of the Germanic-speaking Gothic tribes (Along with the Visigoths and the Ostrogoths) who ended up moving during the Migration Period to the Crimean Penninsula.
