Your client keeps a list of everyone you follow.  You can choose to what relays you publish that list in a kind 3 event, When you go to a new client it pulls that list from the relays so it knows who to follow. This can cause an issue if the client you move to cannot find what relays your kind 3 is published to, finds an old list. Or publishes an event overwriting it as may have happened here. 

https://github.com/nostr-protocol/nips/blob/master/02.md