The roots of that big tree in your yard may be infiltrating your underground plumbing right now. If the species is shallow-rooted, it's even more likely.
Tree roots do not wait decades to cause trouble for your septic line. Given moisture, nutrients, and even a small opening, ...
Tree roots naturally grow into sewer lines as they are a source of additional water and nutrients. Some signs of tree roots in sewer lines are sinkholes, soft spots in the yard, slow drains, gurgling ...