Hi Aggie
This really could be a number of things, but there's a few places to start looking.
First, I would jump into your webmaster tools and see if you have had a spike in errors, which you can find in the traffic tab. If the Googlebot is seeing 404 or server errors, your users might be getting the same thing. Look for any spikes in those graphs and see if there is anything that might put a user off.
Next, I would look at segmenting your analytics data. Are you looking at all of your traffic as a whole? It may be worthwhile to look at the data over time in segments, such as the non-paid search, referral, and paid search segments Google analytics provides. It may be that there is one particular traffic channel that is driving the "bad" traffic - such as PPC ads that aren't converting or referral traffic from a new partner. For instance, one of my clients places a banner on an affiliated website with its own tracking URL. We saw traffic increase over a number of weeks, but bounce rate, dwell time and pages per visit fell and it turned out that the users referred from the banner were, by in large, bouncing straight off. After running it for a bit longer, we decided to remove the banner as it wasn't driving relevant traffic. If you see anything unnatural in these segments, you might be able to identify the cause.
Similarly, have you started ranking for any "educational" terms lately? By that I mean, have you started ranking for terms like "what is [keyword]" or "[keyword] explained"? If so, you may be getting traffic for the terms, but because your landing pages have served their purpose, users are arriving, reading, and leaving. Not a lot you can do in this case to improve pages per visit, but you could be able to capture some lead data by adding an email signup to those landing pages.
You should also have a look at your user journeys in Google analytics, which you can find if you go to Audience -> Visitors Flow in the left hand side navigation. See if you can see a pattern here - are there particularly pages that users are going to that has seen a jump in exit rate over time? Look at those pages - has any HTML/javascript code gone wrong? Is anything broken on those pages? Are they, in fact, just low quality pages that you could improve? This might also reveal something.
Don't be too worried though if you can't see anything standing out. Sometimes these things happen and recover - other times user behaviour simply changes. These methods above might be able to point out any faults you need to address or any improvements you could make. But if your traffic and conversions are steady, pages per visit decreasing isn't such a bad thing. In fact, if conversions are steady (how ever you measure them), one could argue that users taking a shorter time (and journey) to convert show that they're trusting your brand and website more to make the decision earlier.
Hope this helps Aggie!