In Australia I have always respectfully told the truth. It is no secret some companies have terrible management and IMO It can come across well to the type of employers you might want to work for. In this situation I might say something like - "Well I had some great ideas on ways that we might improve our technology stack but unfortunately the manager there seemed threatened by my ideas and shot them down without further discussion. In the end this friction seems to have lead to them deciding to let me go. I learnt a lot from my time there and look forward thriving in a more constructive environment!"