New headlights bulbs in a effort to have brighter headlights actually run hotter.
As with anything that generates heat, it is not only the heat generation that could cause a problem, but the lack of heat dissipation. Take for example a computer power supply, we know it generates heat, that is expected, it becomes a problem when the fan stops and the heat increases.
So, if you did not change the bulbs, what changed the staus quo of heat exchange.
Most headlamps assemblies on the taurus are plastic, even the lens.
If both high and low bems are on simultaneously that might do it.