Osgood Schlatter's disease (or Osgood-Schlatter's disorder) is an 'overuse' condition, where the attachment of the patellar tendon to the tibia bone becomes stressed.
The attachment area of the patellar tendon is called the tibial tuberosity (or tibial tubercle). The bony bump is actually an apophysis - ie a small distinct growth plate - so repetitive abnormal traction here can trigger bony enlargement of the bump. This is thus called a traction apophysitis but is more commonly called Osgood Schlatter's disorder (or 'disease').
There is usually no single actual traumatic incident that starts it off, but rather just repetitive traction.
The condition is very common and it is usually the highly competitive youth that suffers from these flare-ups.
Dr Lars Blønd - 2017 - Arthroscopic excision of an Osgood Schlatter ossicle