There is always the scape-goat that Eru has complete freedom and can do whatever he wants, when he wants. The valar don't have to know everything; they can claim they have the right to be surprised too like the rest
On another note, in the discarded Guilfanion's Tale in BoLT I, the wizard Tu falls into fear of Manwe and even of Iluvatar when he learns that the elf Nuin has stumbled upon the still-sleeping first Men. We don't know why Tu feared Eru; maybe Manwe didn't tell the elves exactly because it was Eru's will - in Osanwe-kenta it is stated that Manwe "would do this or abstain from that in accordance with the [injunctions of Eru], always". Moreover, in the Atrabeth, it is stated that "the designs of Eru ...governed all the operations of the faithful Valar".