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:

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$