Nodes take the first block they receive or which ever block is winning in votes as observed by vote traffic coming off the network compared against the set of stake holders currently in the ledger.
So at the very least, nodes which are syncing would accept my fork as truth? Also possibly online nodes if my fork was presented within some window of time?
Sounds right I think, bootstrap poisoning is one attack vector. One way to address this would be to bootstrap to your representative or whoever gave you your wallet code. Presumably if you trust them enough to download and execute a program that manages your private keys, you can trust them to bootstrap you correctly.
Another option is to bootstrap to someone you're interested in sending to, a merchant perhaps, because in the end the only thing that matters is if they'll accept your payments as valid and they'll tell you what they're accepting through bootstrapping.
Another problem would be if almost everyone simultaneously changed their representative while forks were occurring. This would would require every account to be online and simultaneously signing a change to their representative; interesting but I don't see that actually happening.