1. There is very little context for this graph, and "Average Hours Worked by Persons Engaged" is not defined very well.
2. What happened in Germany in ~1975 that made them suddenly start working?
3. Where is this data being sourced from?
I think it may have been the oil crisis in 1973. During the crisis, industry did not have enough work for its workforce. Instead of mass-layoffs, they tried to shorten working hours and cut wages accordingly. I think in 1975 the situation had normalized again, so that working hours increased again.
So, rather than an increase in 1975, it's probably a decrease just before.