Does Marketo change a lead to 'Marketing Suspended' if there is no activity for a long time?

As far as I know, marketing suspended is never used by the system. It can only be used by you. If the reason is blank, then it probably never got filled in any way.

Marketo uses Email Suspended for system blocks.

Check if you have a flow that is suspending leads.

Hey Mandy, like Josh, I haven't seen this before. If someone (or something) is suspending the leads, it won't necessarily add a marketing suspended cause.

One way that might be helpful to find where this is being written, is either in the activity log of someone who is Marketing Suspended within the last 90 days (data value change details are removed after 90 days), or by finding where the Marketing Suspended field is used within the Admin > Field Management section. 

Hi Mandy,

Try taking a sample of the leads and find them in your lead database and take a look in their activity log. You should be able to find the action that marketing suspended the leads and what smart campaign/program it happened in. :-)

That flag is only set by the Marketer, through a campaign or the like, never automatically by the system. I agree with the comments here on picking a few leads and looking at their activity to figure out who/why they may have been put in that status.