I think Ag2CrO4 becomes soluble in HNO3 solutions since the Ag2CrO4 reacts with the HNO3 in a double replacement reaction to form AgNO3 and H2CrO4.
Ag₂CrO₄(s) +2HNO₃(aq)⇒2AgNO₃(aq)+H₂CrO₄(aq)
I have also heard people say that it has to do with chemical equilibrium and Le Chatlier principles since the acid could react with the CrO4 to make Cr2O7 and H2O. Which would allow more Ag2CrO4 to dissolve since CrO4 is being taken and the equilibrium expression looks like Ksp=[Ag]²[CrO₄]
Ag₂CrO₄(s)⇄2Ag⁺(aq)+CrO₄²⁻(aq)
2HNO₃(aq)+2CrO₄²⁻(aq)⇒Cr₂O₇²⁻(aq)+H₂O(l)+2NO₃⁻(aq)
(if you combine the two above reactions it sort of looks like the double replacement reaction I said at the beginning. It could be possible that H2CrO4 decomposes into water and Cr2O7 which would tie the two ideas together but I don't know if that actually happens)
I hope this helps you answer your question.
