I have seen something like this before - a large shareholder...

I have seen something like this before - a large shareholder looking to exit their holdings hanging over the stock.
It happened to a coal mining friend of mine.
Nathan Tinkler held up to 21% of Whitehaven Coal. He was declared bankrupt in February 2016.
His holding was overhanging the market and depressed the share price for a long time despite the fundamentals of the business being very good.
In February 2016 the Whitehaven share price was trading as low as 0.35. It was extremely stressful for Whitehaven management and shareholders, but they had faith in the business. My friend used the opportunity to build a bigger holding and made 10x within a short period.
Once the Tinkler overhang was gone, by October 2016 the price was trading around $3 and has traded as high as $11 since.
I don't know why Cumulus (assuming it is them) is selling. It could be for many reasons.
Once they are gone it is a reasonable hope/expectation that the share price will improve to reflect the real value of the company.
Of course that assumes GBR management proceeds with the plan to mine Ironbark in the near future to realise value in the ground.