It's caused by the air conditioner compressor cycling in.
When you use the windshield defroster, by default the A/C runs also, to
dehumidify the air.

There is even has an explanation for it:

Headlights & Dash Lights Dim With Electrical Load

NOTE: This article applies to all Honda models
except Passport.
Soon after the change from daylight savings time to
standard time (the last Sunday in October), Tech Line
gets numerous calls about flickering or dimming
headlights and/or dash lights when the brakes or turn
signals are used.
This flickering or dimming happens when the battery
is supplying most of the current for the electrical load
instead of the alternator (something a customer is
more likely to notice when their headlights are turned
on).
Since the early ’90s, Honda vehicles have come with
an electrical load detector (ELD) unit in the
under-hood fuse/relay box. This unit allows the
ECM/PCM to regulate the alternator (switch it
between high output and low output) to provide the
best combination of fuel economy and electrical
system operation. The ELD sends a signal to the
ECM/PCM that’s proportional to the electrical
demand. The ECM/PCM switches the alternator
between high output and low output depending on
several factors, which include electrical demand,
battery charge level, and the driving cycle. When the
alternator is in low output, the engine load is reduced
and fuel economy improves.
If your customer complains about this flickering or
dimming, tell them about the ELD unit and how it
works. Unless the engine won’t crank, or the vehicle
has other symptoms of a weak battery, chances are the
flickering or dimming isn’t an electrical problem.

--Gene