I kind of want some context for the use of the term migration when talking about the Viking Age, or Anglo and Saxon tribes conquests of England. I really don't get why they are called migrations, as opposed to invasions.
Could you provide some context? Nothing I've read about viking-age settlement in England refers to it as migration after about 865.
The Anglo-Saxon migrations appear, from all the archaeological evidence available, to have been peaceful movements of very small groups of people (several families establishing small new settlements, not large armies conquering territories). So in this case, the language is chosen to accurately convey the current scholarly view: that we're looking at several centuries of people moving / migrating, but probably aren't observing the effects of a violent invasion or conquest.
Robin Fleming, Britain after Rome, gives a good summary of this view in her book's earlier chapters.
What the germanists call the migration period or die Völkerwanderung, the Greek and Roman historians, amongst many others, clearly labeled as a period of barbarian invasion.
Using the term German or Scandinavian migration or Völkerwanderung instead of invasion signals one's perspective on the events. Using the term Scandinavian rather than German or Germanic suggest you are from the Gothic school which holds that southern Sweden was the origin of all the Germans.
See: Well Spring of the Goths by Ingemar Nordgren (2004).