The rain came several days after the snow. Yes, it was a record snowfall, but only barely (1 or 2 cm...). Heavy snowfall is typical for the region. Winter 2005/2006 was similar - and did not lead to a breakdown. 2010 also was challenging.
Problem is that the last couple of winters have been relatelively mild, and memory is short. Also, the climate change narrative (which is often misunderstood: global warming does not preclude cold local temperatures. A warmer atmosphere can hold more water, which means rain and snow events, if they occur, can be even more intense) lead to complacency.
City administration, MVG (Munich Transit) and DB management just were ill-prepared and/or panicked.
Munich no longer has adequavate snow capacities - many snow plows deemed unnenessary and decommissioned. In the 1980s, during heavy snowfalls, a task force of city officials would call up the unemployed, students, etc, give them shovels and ice picks and let them earn a handful of Marks per hour. This doesn't happen any more.
DB had stopped all trains in the early morning, just after the snowfall had begun - it was less than 15cm at that time. That was something that any train can handle. Had they kept running, frequent train traffic would have cleared the additional snow. Instead, they let it build up until it no longer could be handled and snowplows were needed.
Same with the catenary. By stopping traffic, they allowed wet snow to build up on the catenary, eventually shortening the isolators. Then the engines/EMUs sensed the power loss and lowered their pantographs, in several instances taking the catenary with it as the panto had frozen to the wire during hours of standstill.
And why did DB Netz stop traffic? Because there had been several trees collapsing under the load of the snow and falling on the rails... And that is criminal negligence. Until the late 1990s, DB owned a smalltalk strip of Land surrounding the tracks. Trees in this strip would be monitored and trimmed to a size that they could not endanger rail traffic, weak trees would be felled. The workforce for these jobs had been reduced to quasi non-existent (which is why spring and autumn winds also cause carnage and chaos every year).
The point is not that conditions were not challenging. The point is that the shutdown was completely avoidable. If I am paying the second highest taxes and the highest social security contributions in the developed world, I demand something back. The snowfall was just one in a line if many things showcasing that that which was once was taken for granted does not work any more. Our country keeps failing. And far to many people seem to be okay with this.