Most likely (as already stated) you are temporarily near a more powerful source of RF wireless transmissions, and in the same freq. as your bike computer. Lots of homes have wireless now for computing as well as other devices.

Why not both bikes? The wireless bike computers most likely are set to transmit and receive on specific frequencies, and the manufacture probably has a number of frequencies set in their individual devices. If all of them of that brand used only one frequency, and you rode close by the other computer, you might get the data from the wheel on the other bike. I don't know what the reception range might be, so this is speculation.