The ports auto-negotiate with the devices they are connected to as to the best speed they can support. A temporary problem with a device may have screwed-up the negotiation and the port dropped to 100Mb/s.
Note, it is also possible that, if the only device "visible" to the port at the time was something with a 100Mb/s network card, (most often a TV box, a separate ATA or a printer), that would also cause the port to think 100Mb/s was appropriate. Unplugging and re-plugging would trigger a reset of the port.
Then again, it could just be faulty. As I said, muck on one pin in the socket may have the effect of dropping it to 100Mb/s.
