This is definitely abnormal. It looks to me that your iMac's BT or keyboard is malfunctioning; alternatively, you may hit the Option key too late. That said, my bet is on a malfunction, possibly brought about by interference from other RF sources, or some degree of screening or signal attenuation between BT-linked components. Unfortunately, this doesn't necessarily help in fixing the issue, but you may be able to further pinpoint it.
