which  mundane planetary aspects were most responsible for the riots in 2020 and when will they conjoin again?

Transits are of little importance. You need to look at Ingress charts done for A particular location:

Mundane (worldly events, as opposed to individual psychological cycles) is done using “Ingress charts”.A chart is set up for the MOMENT that Sun enters any of the 4 Cardinal Signs (Aries or Cancer or Libra or Capricorn), set for the particular longitude and latitude of a particular location.  You then interpret the chart according to set rules of Mundane astrology.Relevant factors for your questions about riots would relate to• Moon (which indicates, among other things, the mood of the people)• the 1st House/Ascendant (which indicates the population)• Saturn (which indicates, among other things, restraining laws)• Uranus (which indicates, among other things, riots)• Pluto (indicates among other things, the masses, scandal)• The 6th House (which indicates, among other things, the working class, the police)If any of these areas would experience trouble during the 3 months of the Ingress chart, you would expect to see these planets, or the planets ruling these houses, being “angular” (conjunct any of the 4 “angles” – the Asc, the IC, the Desc, the MC) AND being negatively aspected by other planets in the Ingress chart..An Ingress chart for Minneapolis (covering late March to late June 2020) would be done up for March 19, 2020, at 10:49:29 PM, CDT for Minneapolis (Longitude 44N58’48”, Latitude 93W15’49”).Such an Ingress chart reveals:Moon (mood of the people) is angular (predominates). It is in the 3rd house of (among other things) the media. It is opposite the Midheaven of the government in power. But the only hard aspect it has is a square to Venus, which Is not serious. You would expect to see it conjunct or badly aspecting Mars or Pluto or Uranus .. and it doesn’t).For the DC insurrection at the US Capital on Jan 6th, the Capricorn Ingress chart for DC is for Dec 21, 2020 at 5:02”12 AM EST, Long. 38N53’42” and Lat 77W02’12”.In ths ingress chart, Moon (the mood of the people) is conjunct Neptune (hysteria, lies), conjunct the IC (the opposition party .. in this case the losing party). While we might have possibly predicted riots from this, it would be no surprise since Trump had been saying for months that the ONLY way he could lose the election would be if it was stolen (despite the fact that never, during his term in office, did he have the majority of the voter’s approval).I find mundane astrology and Ingress charts to be very entertaining IN RETROSPECT, but they are not very good at predicting things.  But sometimes they surprise:HOWEVER, if we look at the Ingress chart for Sep 23, 2019, 3"50:04 PM AWST in Wuhan China (first Covid case reported during the span of this Ingress), we see Aquarius on the 1st house of the people .. Uranus (ruler of Aquarius) being the unexpected and disruptive.  Moon (the mood of the people) is in self-protective Cancer in the 6h house of the people’s health. And Moon is involved in a very strong malefic T-square:Moon opposite Saturn (repression, fear) which is conjunct Pluto (often found with disasters). Both Saturn and Pluto in the house of secrets. These 3 planets are all square to Mercury conjunct Venus in Libra in the 8th House (death, among other things) .. Mercury and Venus both having to do with activities that would spread a disease). WOULD I predict a pandemic from this Ingress chart? No. But, as I said, Ingress charts are interesting in retrospect. Useless, but interesting.And every Ingress chart is unique to that time and that place, so I cannot tell you if there will be any astrological influence prompting future riots. But politically, it depends on how many more deaths there are from shooting, and what verdicts and sentences are handed down.