What reasons are there for this 'dancing'?

Any ideas on what would cause this kind of fluctuations?

Typically that is caused by aggressive off site SEO.

You'll probably be OK but I would probably slow down anything that was aggressive towards my site.

Such serpdance are normal for me.. Specially for new sites

Check if the peaks of this "dance" as you called it, may happen when you post something new or when you promote your page in a more aggressive way, this could be the cause. When there is new content uploaded and you share it, people will overcrowd your website and this is why it looks like a sudden boost. Also check your site's bounce rate, this will provide you with more information about those inconsistencies. If you have a big bounce rate (80-90%) means that people didn't stay too much on your website and that could be a possible explanation for the downs of this line, because during those days no one or very few people accessed your site.