One of the hypotheses offered to explain migration patterns is that rather than everyone processing in a group like a football crowd crossed with the Clampetts, small numbers of explorers actually go to the absolute limits of their known world, and those that follow on do so in a kind of leap-frogging pattern.
Part of this might be have been due to migrants claiming land and refusing room to migrants following on behind.
These hypotheses are based on observations of modern phenomena, for example pioneers in the Midwest.
Then there are factors that we can only guess at, but which have driven migration in historic times and could have operated in the distant past - for example, inheritance patterns, and religious belief.
Welsh inheritance patterns meant numerous sons were left with no useable farmland so had to migrate to survive, or become slaves (5th - 12th centuries AD and later). The Pilgrim Fathers trek to the New World is a good example of religiously-driven migration to a distant and unknown destination.