The formation of dust is the usual reason for these sudden dips, and that is consistent with the reddening as shown in the recent photometry, but what about the big humps when it was bright? Is that dust related too or is it an actual brightening of the disc? V1405 Cas has these humps too whereas other novae (V1674 Her for instance) don’t.
This recent paper considers the origin of this flaring phenomenon – it’s actually about another nova, but they generalise to all of these slow, dusty systems. However, they state that it’s still an open question -and there may be multiple scenarios in different systems. They favour multiple episodes of mass ejection. They propose that the photosphere expands during a flare – the flares produce shocks which lead to high energy gamma-rays and dust production, others due to instabilities in the accretion disc around the white dwarf created by enhanced accretion stimulated by heating from the secondary star.
The authors of that paper (which include amateurs) make plea for further studies of other novae, so it is good that you and others are following this nova.