When did the second detection of termites occur? Was an actual nest found? Was there just frass leaking from behind the walls or were reproductives found on the walls?
Fumigations are a whole house treatment. Whole house treatments are more likely to kill the hidden, unfound infestation. If the termites appeared too soon after the fumigation, you have to wonder whether the fumigation failed.
A fumigation does not prevent termites from re-infesting. This second infestation could be a new nest of termites. An injection might be appropriate, as the nest should be small, since the previous larger infestation should have been killed by the original fumigation.
Injections of insecticides are spot treatments. Since termite infestations can be hard to find, you have to consider whether a spot treatment is good enough to eliminate your infestation. A company that is using spot treatments only, could be getting complaints because spot treatments can miss infestations. Consider the nature of the complaint.
This website discusses termite species:
Termites101.org
This website discusses fumigation vs spot treatments:
UCIPM Drywood Termites
This website discusses termite IPM:
UCIPM Termites