Trackmod as the name suggests tracks the modified pages of the existing tablespace which qualify for a incremental/delta backup. If you create a new tablespace you have to take a full backup before an incremental one.
I have a very simple explaination for this probably other db2 gurus might have a much detailed one i am not sure.
Trackmod is to track the modified pages of the existing tablespace. So a new tablespace records would not be collected in the previous full backup and your backup if successfull can be in consistent. However a tablespace dropped does exist no more. So a full backup or an incremental backup will still be successfull and still the image would be a valid backup image.
Trackmod tracks the modified pages of all the tablespace and when the tablespace does not even exist it takes it as good as no modified pages and still goes ahead with the backup which would be a successful one still.