They previously built a wall. Siege preparations confirmed.
Its highly likely they are preparing for large scale riots which cannot
be stopped by security and therefore require material defenses.
https://www.businessinsider.com/white-house-anti-climb-wall-protests-2020-7?r=US&IR=T&op=1 ("anti-climb")
Wall and landmines will allow defending with much less personnel.
However it doesn't make sense to stay there. Its likely Trump will be evacuated before the threat materializes. The defenses being setup are to buy time in uncertain situation, basic military preparations for defense.
Trump is preparing for long-term civil unrest and lack of security.