Reasons for calling a patient's disease "progressed":
I found a good paper on progression events - albeit one that includes some non lung cancer patients (about 2/3 are lung cancer) - which notes that:
"Table 8 presents the distribution of events in short-term PFS by reason: the majority of progressions occur due to new lesions being detected, either with or without growth in target lesions that quali?ed as progression."
And FWIW this agrees with both my recollection and the discrete data I can find in other more focused (one cancer type, one treatment type - but much smaller population) studies.
Note: so far the more focused data is:
Melanoma treated with dabrafenib - no progression caused solely by new lesions, but 57% caused by simultaneous target lesion growth and new lesions.
Note that one thing that is NOT clear is that everyone uses the same counting system - e.g. the first paper cited clearly counts a progression with both causes as a "new lesion progression", but I am not sure whether that is standard. In fact I suspect it is not - and may, perhaps, be why your data sounds so different?