Why I Became #1?

Hey,

I run a couple of sites in the same niche.
I have my main site that generates the majority of the income because it's been the one I have promoted the most over the last couple of months.

About a month ago, the other site that is basically used to funnel people into my main site was jumped to #1 for my keyword (only 3 words).

Which is holding steady.

Naturally I'm very happy with this outcome but I am curious, I have spent a while building up my back links on my main website and it currently stands at position #4. But I have never spent time promoting the other site using SEO (The current traffic referral percentage from Google for the site is 78%).
This is an ad that I inserted!
The site is relatively old now and I was just wondering if anybody had any insight as to why the site would jump to the top of the rank all of a sudden (and stay strong for just over a month).

Thanks.

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