Why does the key for a 1997 BMW 740i set off the alarm when trying to unlock the vehicle?

Why does the key for a 1997 BMW 740i set off the alarm when trying to unlock the vehicle? We unlocked the car with the remote key so that we could get it jump started it. In the process of pushing the button to unlock the vehicle it set off the alarm also. We then tried turning the alarm off with the key but the alarm would not disarm, the doors would still lock and unlock with every press of the button as we desperately tried to disarm the alarm. After a many tries we finally got the alarm to stop. Could you tell us what caused this to happen and what we need to do to keep it from happening again? We still haven't been able to jump start the vehicle because we are afraid that we will set the alarm off again. Please help us. Thank you.