Ah. Why would that happen?
If I recall correctly, each peer's memory pool is cleared on exit. Therefore, as nodes go offline (system restart, daemon crash, etc.), the transaction gradually disappears from the network.
Or it could be because blockchain.info is bugged. I sent a 0 fee transaction (that normally required a fee) using the NFTF fork, and it got 95%+ propagation. After 5 minutes, it dropped to 1%. It still got confirmed in ~6 hours.