Phenomenon of equal prices without price collusion
I know you are at pains to say you are not looking for 'collusion' as an answer, but what you describe sounds very much like tacit collusion. Here's a quote from Competition Policy by Massimo Motta (pp138–140):


  I briefly characterise the concept of collusion from the point of view of industrial economics. Note that here I will not use the term "collusion" as a synonym for "collusive agreement that should be outlawed". Indeed, one of the main themes of this chapter is that whereas in economic theory collusion is defined as a market outcome (i.e., "high prices", in a sense to be specified further below), anti-trust authorities and judges should consider as illegal only practices where firms explicitly coordinate their actions to achieve a collusive outcome.
  
  [...]
  In economics, collusion is a situation where firms' prices are higher than some competitive benchmark. [...] In any case, in economics collusion coincides
  with an outcome (high-enough price), and not with the specific form through
  which that outcome is attained.
  
  [...] It is important to stress that [colluding firms need] not talk to each other, neither directly nor through intermediaries: collusive prices will arise through purely non-co-operative behaviour of the sellers. In other words, if detection of deviations is rapid, and if (market) punishments of deviations are likely and credible, then tacit collusion can arise: firms do not necessarily have to talk to each other, let alone agree on complicated schemes, for a collusive outcome to be sustainable. All that is needed is the awareness that a deviation will be identified, and that a "punishment" will follow.


The 'punishment' that Motta refers to is exactly the idea that, if one firm cuts its price to break the tacitly collusive status quo, other firms will 'punish' it by responding with a price cut of their own.

Sometimes, tacit collusion is known by the alternative name conscious parallelism to reflect the idea that firms are consciously mimicking each others' strategies. Particularly among legal scholars, the term conscious parallelism is used to mean coordination of prices without an agreement, whereas the term collusion is reserved specifically for the case in which an agreement has taken place. Among economists, as Motta explains above, both practices are known as (respectively, tacit and explicit) collusion and the conscious parallelism terminology is less common.