I find it helpful to separate the observations from the model and interpretation. The observation was that there was a curious linear trend between distance from Earth and velocity. This was simply an observation made on galaxies that we could see. Such a trend would not be expected in random data. Indeed, it would suggest that we might actually be the center of the universe, as everyone expands out from us.
Of course, that is not the popular interpretation. We don't like to assume we are that special. Instead, it was identified that modern relativistic theories permitted space to expand. If space itself was expanding, then a linear relationship like we see from Earth would be seen from everywhere. This is the preferred interpretation: $v=H_0d$ is seen as evidence that space itself is expanding (and taking matter along for a ride).