Removal of credit link

[rms]

I had an idea that credit link never gets deleted, only value of balance and limit gets modified. However, I found that initially some credit links were present in the network but later on got deleted. i.e,

1. Timestamp : Aug 2013
{"result":{"account":"rBXuPiEUXbRwWqLv4wkMLDKpQWVDviZBzm","ledger_hash":"AF803E7342D02CF9399856B7A921EECAEE717740BCB0EED01DD5E7F4DD7DEA56","ledger_index":1841211,"lines":[{"account":"rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B","balance":"13.492063","currency":"USD","limit":"0","limit_peer":"0","quality_in":0,"quality_out":0}],"validated":true},"status":"success","type":"response"}

2. Timestamp : Dec 2013
{"result":{"account":"rBXuPiEUXbRwWqLv4wkMLDKpQWVDviZBzm","ledger_hash":"E7C5CF4D8FC43C2452704FBA9719E800C53DC5717CC171D05540606A8F732107","ledger_index":3941211,"lines":[],"validated":true},"status":"success","type":"response"}

As we can observe, the node's link with gateway has been removed in the Dec 2013. Any idea which kind of events can lead to this kind of situation? And is it really possible to delete credit link?