Quite often a sewer blockage can be caused by the roots from trees that are not even on your property.
Tree roots will travel great distances to seek moisture and nutrients readily found in sewer pipes, especially during periods of extended drought. Amazingly in some cases the roots will travel from the base of the tree upto 3 times the height of that same tree. So a 15 metre high tree will spread its roots upto 45 metres from the base of the tree.
Check this Youtube video to see how far tree roots travel to find sewers and pipes just like yours!