Read it again, I think it's just math. If there is a certain amount of transaction volume, at a certain bitcoin value in USD, that just adds up to a certain USD transaction value per day, right?
At first I agreed with you, and thought the author only wrote 'appears to be' to hedge their sentence, because they didn't think it through 100% to be sure of their reasoning.
But consider: does the sentence read exactly the same if we rewrite it as,
>Taking these three charts together, the relatively static USD transaction value per day metric in 2014 appears to be the result of the increase in transaction volume being offset by a falling market price of bitcoin.
Not at all!! So the author really is saying something that implies a causative effect: the price is falling, so people how need a certain transaction amount (in real terms) will transfer more.
This makes perfect sense when you consider the authors follow-on sentences.
So absolutely, the author is making a causal relationship. The author even justifies it (pretty well in my opinion.)
