bg_backlog_size on one node keeps growing on an idle system

I loaded a two node cluster with 13.7M items.
Replication is caught up on both nodes, (total items is double then curr_itmes), but on one node (10.2.1.13) the bg_backlog_size keeps growing, started at around 100K, reached 4M at some point, went down to ~2M and. there is no traffic on that system:


Sharon-Barrs-MacBook-Pro:scripts sharonbarr$ ./stats 10.2.1.13:11210 tap
 ep_tap_ack_grace_period: 300
 ep_tap_ack_interval: 1000
 ep_tap_ack_window_size: 10
 ep_tap_backoff_period: 1
 ep_tap_bg_fetch_requeued: 0
 ep_tap_bg_fetched: 7047333
 ep_tap_bg_max_pending: 500
 ep_tap_count: 1
 ep_tap_deletes: 0
 ep_tap_fg_fetched: 26839455
 ep_tap_keepalive: 300
 ep_tap_noop_interval: 20
 ep_tap_throttled: 2003363
 ep_tap_total_fetched: 33891978
 ep_tap_total_queue: 1867398
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:ack_log_size: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:ack_playback_size: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:ack_seqno: 26871351
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:ack_window_full: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_backlog_size: 1867398
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_jobs_completed: 6636686
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_jobs_issued: 6636686
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_queue_size: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_queued: 6636686
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_result_size: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_results: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:bg_wait_for_results: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:complete: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:connected: true
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:disconnects: 187
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:empty: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:flags: 20 (ack,vblist)
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:has_item: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:has_queued_item: true
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:idle: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:num_tap_nack: 14062256
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:num_tap_tmpfail_survivors: 14062256
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:paused: 1
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:pending_backfill: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:pending_disconnect: false
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:pending_disk_backfill: true
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:qlen: 1867398
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:qlen_high_pri: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:qlen_low_pri: 0
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:rec_fetched: 20234665
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:reconnects: 187
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:recv_ack_seqno: 26871350
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:suspended: true
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:vb_filter: { [512,1023] }
 eq_tapq:r-ns_1@10.2.1.14-ns_1@10.2.1.13-1299541139.347474:vb_filters: 512
Sharon-Barrs-MacBook-Pro:scripts sharonbarr$ ./stats 10.2.1.14:11210 all | egrep "curr|ep_flush|mem|queue"
 curr_connections: 27
 curr_items: 6855859
 curr_items_tot: 13692060
 ep_dbname: /var/opt/membase/1.6.5.2r/data/ns_1/default
 ep_flush_duration: 354
 ep_flush_duration_highwat: 365
 ep_flush_duration_total: 7014
 ep_flush_preempts: 0
 ep_flusher_state: running
 ep_flusher_todo: 108003
 ep_mem_high_wat: 24499716096
 ep_mem_low_wat: 19599772876
 ep_queue_age_cap: 900
 ep_queue_size: 391290
 ep_store_max_concurrency: 10
 ep_tap_bg_fetch_requeued: 0
 ep_total_enqueued: 24570524
 mem_used: 19602399852
Sharon-Barrs-MacBook-Pro:scripts sharonbarr$