Fair enough @jcmleng - in my experience, when Vanguard have done chunky buys it's because either (a) the company has been added to an index that Vanguard track with one or more ETFs, or (b) Vanguard have created one or more new ETFs that hold that company, or (c) Vanguard have added the company to one of their existing ETFs such as a thematic ETF (SAAS, IT or growth/income, that sort of thing), or (d) Vanguard have rebalanced one of their existing thematic ETFs and decided to increase exposure within that ETF to that company, often because that company's own share price has increased so they have a larger market cap.
When an ETF holds a company based on its market cap, the ETF will generally buy more of that company as the market cap rises, particularly when that company is moving up the rankings within their own sector, i.e. their market cap is climbing faster than their peers. In other words, when passive fund flows are involved (ETFs), a rising share price can be a self-fulfilling prophecy. The higher the share price, the larger the market cap, the more stock the ETF(s) need to hold in relation to that company's peers, the higher the share price rises as a result of that buying... something that has probably occurred with HSN whose SP has risen from $4.86 on 10th July to now above $6/share.
HSN haven't been added to an S&P ASX index since 2007 (the ASX300 Index), so I'm thinking it's most likely related to HSN's share price rising after their guidance upgrade announcement on 14th July causing Vanguard to buy more shares to maintain HSN's market cap position within their ETFs (playing catchup).