I agree with the others, this is mostly likely acused by a temporary change int he way Google si viewing your site. Possibly by a link apeparing/disappearing or possibly from an algorithm change. I ntoice this on my sites all of the time.
It is, of course, also possible that the other sites reacted to your ranking increase, and started to do more SEO and now rank above you because of that - but this is highly unlikely in my experience.
The best next steps is to keep building your link profile. Ensure you get good quality links and Google will recognise this and start to attribute more rank.
One point that is worth mentioning, is do you think that the site you've sent should rank at #2? Taking a look at the SERPs that are more relevant pages to rank at #2 then the sub domain in question. The top leveld omain of your site is in fact #1. Maybe you need to increase the on page relevenecy of your sub domain to the keywords you're ranking for?