Signer sometimes reverts to AXFR and signs full zone again

Signer getting notifies from unsigned zone which is updated frequently - every minute.
Most times the signing stats show that an IXFR has been performed and only the new records processed:
Dec 3 01:31:58 signer ods-signerd: [STATS] example.com RR[count=7 time=3(sec)] NSEC3[count=0 time=0(sec)] RRSIG[new=1 reused=16470 time=15(sec) avg=0(sig/sec)] TOTAL[time=138(sec)]
BUT, sometimes it seems to do an AXFR and re-process all records:
Dec 3 01:36:11 signer ods-signerd: [STATS] example.com RR[count=4294967293 time=3(sec)] NSEC3[count=0 time=0(sec)] RRSIG[new=1 reused=16470 time=14(sec) avg=0(sig/sec)] TOTAL[time=132(sec)]
Note the RR count is huge - every record processed.
This happens approx 1 in 20 signings. Will upload logs with verbose=5. ANy more would show all records and be huge!