@fbquestion,
Ah, now I get it.
FB is getting a social signal from Page #2 watching Page #1. It's decided that Page #1 must want to (reciprocally) watch Page #2 because it's using that signal and determining that the pages are similar enough that it would make sense for you to watch it.
See, FB generally works on the basis of algorithms. The site is so enormous that it needs to do almost everything in an automated fashion.
Hence there are, let's say, 50,000 gardening pages (this might even be a low estimate, I might add). FB won't recommend all 50,000 to you to watch. Instead, it attempts to determine this by certain social signals, whether that's a page's location or its listing of members or even the captions on the photos it has posted. It's in FB's best interests for the delivery of "pages to watch" to be as good as possible. Hence out of the 50,000 pages, if you're in Boise and you only sell petunias, it's going to get stronger signals from pages where Idaho is the listed location, or petunias are one of the terms used in the majority of their photo captions.
A very strong social signal is sent out when page #2 follows page #1. FB's just weighing all of these with its algorithm (their algorithms are rather closely-guarded secrets; I'm just deducing this from observed behaviors) and going with a suggestion for a page where the social relationship is being signaled to FB that it's strong.
A similar thing occurs with the suggested friends listing. You're served up people who you have friends in common with, but you're also shown people who attended your High School within about 5 years before and after you graduated, people who list your town as their home town, people with similar interests, etc.
But FB's main product is delivering a semi-captive audience to advertisers. They do like to make things better for pages because page runners, big and small, are some of the biggest purchasers of advertising on FB.