S7-300 and the Memory Card backup.

I have a machine with a 315-2 DP processor.

A while ago something happened (I wasn't around, offshift, etc,) and the machine stopped working properly during a motion control portion of the cycle. After much troubleshooting and guess and check work, we found out that all of the movement data was reset to 0 somehow.

Well, we got it working again, and I'm just curious as to what may have caused this problem.

Best idea I can come up with is that someone hit the MRES switch on the processor (Thinking it was just a reset) and it cleared the memory, and then loaded the saved program from the memory card.

Is there any reason why these values in the DB weren't stored in the memory card along with the program (other than the program on the card not being up to date)? I guess what I'm asking is does the memory card store everything? Should it be able to store these important values and load them up properly if there was ever a memory reset?

In principle the actual values should be stored upon shutdown. Siemens calls the PLC "maintenance-free" as there is no backup battery.

But I have experienced, and others too, that the actual values (all or just some of them) can be reset to the initial values, or to zero, or to some random value.
I think this can happen if there is a lot of noise, or if the PLC is cycled too fast from ON to OFF and back to ON again.

Datablocks initialised

I also had it twice that the datablocks were reset tot there initial values. Once in a situation with shortcircuit problem in the 24 V (which could be similar to the fast on and of the plc as Jesper said) and once in an older machine when the put the power back on after every weekend (which may be because of the supercapacitor not funictioning anymore as it should).
I alwalys try to do a ram to rom at the end of my commisioning, so the values are stored on the mmc. This has the advantage that the mmc can be placed in a new plc directly with good parameters (from the commisioning) and that in case of problems like you had, you can at least with parameter values of the commisiong.