A common issue that many evol-cancer researchers are focusing on is the effect of micro-environment on cells when talking about competition among somatic cells and their strategies, but I don't think it's the most essential one to be emphasized.
An intuitive argument is, if a bunch/lineage of somatic cells have adapted to stable environments, how come they could quickly switch to another set of strategies that fit the new fluctuating environment? It's hard to accept such a scenario UNLESS this plasticity (or capability of switching strategies) itself is also an outcome of evolution. As a result, the focus should be what have made that plasticity possible, (only) based on which we can talk about how cell strategies are manipulated by the micro-environment.
It's like a classical disputation between the niche theory and the neutral theory in community ecology. I won't talk too much on it because the disputation may not have finished yet. The point is that under niche theory you know definitely some environmental factors are making influence but nothing helps you find what they are and how they function. Now the same difficulty is seen here in evol-cancer research.
Whether it's my prejudice or not, I would prefer to focus on internal causes before any external causes are considered.