What you have to realize is how important Security is to Shopify. We are a trust based business to an extreme extend. We host the livelihoods of hundreds of thousands of other businesses. If we are down or compromised all of them can't make money ( as some of you saw during Black Friday, to the tune of $300k+ a minute at times ).
One of the best ways for us to augment our internal security team is to work with the white hat community. This was a pain before Hacker One but now is significantly easier.
One challenge is that Shopify (still) hasn't really got the profile in the tech industry that a lot of Silicon Valley local companies have. This is totally fine by me, but it's means that if a top white hat sits down and decide what to work on, we are not automatically top of mind.
So we decided to overspent as a kind of "marketing" investment. Hacker one is a classical two sided market place. There is plenty of supply of skilled researchers but also a lot of demand for their services. We want to be known for being one of the most responsive companies and also pay top dollars for top findings.
So the basic idea is that when we launch something new, we 10x the payouts to bootstrap the process of familiarization. We also provide a very convenient local environment for doing the work in. It should be more fun and more lucrative to make Shopify related discoveries then other companies. After this initial period we then reduce the payouts somewhere slightly above community standards. Its all just business 101.
Internally we are actually thrilled how the shopify-scripts/mruby program went. Most (all?) of what was found would have been caught by our sandboxing but we don't want to rely on this. As everyone who does security knows - lots of exploits, even if superficially contained, can sometimes combine into "the big one".