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This ISS status update, dated June 28, explains why the piece of space junk that flew past the station that day was such a surprise. (Hat tip to NASA engineer James Fincannon for pointing this out to me.)
Last night at ~6:00pm EDT, NASA Houston FTC (Flight Control Team) received notification of an upcoming “red threshold” conjunction of the ISS with a piece of orbital debris (Object 82618, UNKNOWN), with a TCA (Time of Closest Approach) this morning at 8:08am EDT, – which was too late to begin planning for a DAM (Debris Avoidance Maneuver). Therefore, FTC and crew made preparations for crew sheltering in Soyuz 26S & 27S. PC (Probability of Collision) at last tracking fix (7:20am) remained in the Red box, at ~0.003, with a miss distance of 0.25 km radial, 0.375 km downtrack, 0.570 m crosstrack. The necessary reconfiguration procedures (USOS hatches closed, etc.) began 1.5 hrs before TCA (6:38am EDT), and the six crewmembers ingressed their Soyuz vehicles. At 8:08am the object cleared the ISS with no impact, and shortly thereafter the crew was given the Go for returning to the ISS. [The late notification occurred because of the high air resistance (drag) of the object (~175 times higher than ISS) which made its trajectory very sensitive to small errors in atmospheric density predictions at the current solar flux. Due to the high drag, there is no chance of a recurrence of Object 82618).] [emphasis mine]
In other words, the piece of junk was probably something like a piece of insulation, very light but with a large area, much like a sail. Thus, as it flies through the thin atmosphere at 200 to 400 miles altitude its velocity and direction can easily change, making it difficult, if not impossible, to predict its future trajectory.
The good news is that these same conditions mean that the orbit of the object will quickly decay (“due to high drag”) so that it poses no future threat to the station.
Unfortunately, there are many other objects like this in orbit, and they all pose a threat, mostly because of the difficulty of reliably predicting their orbits.