The Labour party left space on the left of Labour and this space has been filled by the SNP, which historically has not been a left wing party. The more a political party moves to the centre, the more likely it is that socialists will look to other parties or stay at home. Once that gap starts to be exploited, it becomes almost impossible to recover.
It destroyed their local councillor base by introducing STV, creating large wards where councillor contact with the electorate decreased, and the change gave seats in historically solid Labour areas to third parties most notably the SNP.
The Referendum was held at the wrong time. A previous Labour Leader wanted it when we controlled the Parliament but was blocked by Westminster Labour. We would have won it easily 10 years ago but we failed to take the opportunity.
The SNP were able to use the "run by London" jibe at us and it was effective in making us appear "less Scottish".
Perhaps the biggest mistake was not having a Labour 'Yes' campaign and allying ourselves with the Tories in the 'No' campaign. We also failed to answer blatant untruths by the SNP in the referendum and by the time we started to do so we were not believed.
Too many aspiring Scottish politicians believed Westminster was more important than Holyrood.
We failed to identify SNP weaknesses and capitalise on them e.g. their support for bus deregulation.
We failed to capitalise on the tension between rural and urban parts of the SNP.