Even though an alternator is rated as 170amp or 180amp unit, I believe it is a "per hour" rating. In other words, when the alternator is running at the proper RPM, it is capable of charging at a rate of 180 amps per hour while maintaining 14.7 volts (typically the voltage implied for overcoming internal resistance in the battery). The vehicle's battery supplies any additional power required above that hourly amperage draw.

This would be especially true in a diesel when the glow plugs themselves place a huge demand on the battery, coupled with the load of the starter turning over a higher compression engine with "stiff" oil. I can remember years ago with my GM 5.7 liter diesel that the startup at 32 degrees F could produce a 360 amp draw between the glow plugs and starter. And the old rule of thumb with gasoline engines was it took twenty minutes of normal driving to restore the energy to the battery that was used to start the engine.

It's quite possible, that depending on what electrical items were on at the time (A/C fans, electric radiator fan, headlights, stereo, heated seats, rear window defogger, etc), the instantaneous amperage output of the alternator was exceeded, dropping the voltage below the point at which the ignition system would work.

2002 x5 3.0 248,125 miles
2004 325i 110,000 miles

Historically: 1981 Pontiac Bonneville 2Dr Coupe 5.7L Diesel 265,000 miles!