After RCT treatment, how did the infection get spread to the jaw?

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As per the radiograph (attachment removed to protect patient identity), your tooth infection had extended beyond the root apex and entered the underlying bone, and it becomes a periapical abscess. The tooth roots are much flared also. Root canal treatment is a conservative approach to saving the natural tooth by removing the infected tissue within the pulp chamber and root canal. Some teeth may have additional root canals than usual. So, there may be chances to miss any canals during root canal treatment. For example in lower molars usually three canals are there, but in some patients, the same tooth may have up to five canals.

In some cases, the apex of the root canal may be very narrow, calcified and curved. So, the orientation of the instrument may deviate from the exact root canal curvature and take another path. Hence, a part of the swellings will be untreated or not cleared from infected tissue. Initially, it can subside with antibiotics, but later infection can be exaggerated after stopping antibiotics. One more reason for the secondary infection is the infected tissue that can be pushed beyond the apex during the procedure. It can also cause a secondary infection. Your endodontist can attempt for a repeated root canal treatment, and wait for a week. If there is no progress in the condition, then go for extraction of the tooth. If swelling of the face is more, then better get the extraction followed by incision and drainage of the abscess done by an oral and maxillofacial surgeon.