Or HaChaim on Numbers 16:19/אור החיים על במדבר ט״ז:י״ט

1 ויקהל עליהם קרח, Korach assembled against them, etc. This tells us that these people did not voluntarily assemble against Moses but only after Korach had put heavy pressure on them to do so. Accordingly, the Torah reports something here that is favourable for this whole congregation of 250 men. On the other hand, the words ויקהל עליהם may only provide the background to the command by G'd in verse 20 to separate themselves from this congregation in order that G'd could destroy them simultaneously and at once. The verse would provide the answer to the people who questioned why the animals had to perish along with their masters. What sin had the animals committed? To this the Torah answered that seeing the animals had obeyed Korach they were considered as if they had been supporters of his.

2וירא כבוד השם The glory of the Lord appeared, etc. The entire congregation experienced this glory of the Lord in order that they would recognise that it was G'd Himself who carried out this judgment and that only He decides who is worthy to serve Him in the capacity of a priest.