Some points need to be considered:
- As stated by Bregalad in a comment, left and right definitions are very relative. Democrats in the US would be centre-right in France.
- And the reality if often more complex than what it may seems. Amonst your examples, in Catalonia, the left-wing ERC reached an agreement with the right-wing CiU. And the CiU is the currently governing party. Often both sides are present.
- In a same country, different "independentists" groups have different views. For example, in France, in Brittany, the main (but not only one!) independence movement is represented by the UDB, which is mostly on the left. Whereas in Alsace, the most notable regionalist party, Alsace D'abord is on the far-right. So it is not a simple rejection of the central government.
I think that the reasons are more historical. In Scotland, even now, the second political party after the SNP is the Labour (left-wing), which was until recently the first one.
It can be seen in local elections: in Berlin, Germany, the left-wing SPD have lead the government since 2002; Baden-Würtenberg has been taken by a left-wing coalition in 2011; and meanwhile, Bavaria is overwhelmingly kept by the right-wing CSU.
Now why some regions/states usually vote on one side, while others on another side reflects their local culture and history and is way beyond the scope of this answer.
But the independence parties/groups essentially reflects that local culture and/or history. Which explains why you observe the mentioned differences.