Aulë was most like Melkor and both had the same desire to create life, independent of Ilúvatar.
Melkor was jealous of him, for Aulë was most like himself in thought and in powers. [...] Both, also, desired to make things of their own that should be new and unthought of by others, and delighted in the praise of their skill.
This and subsequent quotes taken from The Silmarillion by J.R.R. Tolkien
In the creation of the Dwarves, Aulë is the only Valar who we know has the capacity to disobey Ilúvatar. We know that Melkor's skill was in manipulating small weaknesses and doubts between allies, and
others he corrupted afterwards to his service with lies and treacherous gifts.
Aulë was upset when reproached by Ilúvatar for the creation of the Dwarves. It fits with everything we know about Melkor that he would seize upon any seed of disobedience and manipulate it. His unique understanding of the mind of Aulë would have given a way in. Not only that, but
[...] he had secret friends and spies among the Maiar whom he had converted to his cause; [...]
The most important of whom was Sauron, who was of the people of Aulë. Melkor knew the mind of Aulë and had a spy embedded within his camp, would he not have tried to manipulate him into defection?