It depends on the keywords you are using; how many times these keywords are getting used to search for something. This is it. The more times these keywords are in the search box, the more time your hubs are getting crawled. It has no relation with your hub's popularity, votes, or blah blah. The popularity may count when it comes to the point of ranking on the search results page.
Actually, the bots don't read your hub like we do. They read the HTML of your hub. Hence, your title has H2 tag and your hub body has H3 tag; this is why these two things are only needed to get crawled by a google bot.
On the other hand, it sometimes depends on the backlinks your hubs have. Because, the bots surf the web from link to link. Well, It's not always necessary also. Because, when you set a category for your hub, it ultimately works as a redirecting link for your hubs. And if you put up summary, then it gets crawled at a high rate. Because, hubpages use your hub summary to catch the attention of search engine bots.
Your hubs to get crawled, must have a minimum of 1% keyword density. Trust me, they are getting crawled everyday. ;)