Several Big Ten athletic directors have said that they thought that Delany was spooked by the ACC's move on Notre Dame and that he moved to protect Penn State from poaching.
Penn State was seen as an eastern outlier that may be vulnerable to an ACC approach.
Yes and no. You're forgetting that the ND - ACC announcement was many months after Maryland and Rutgers had already announced their intention to join the Big Ten.
The Big Ten originally hoped to be the preferred major sports conference in the Northeast without having a team in that region based on the popularity of Penn State, Ohio State, Michigan and ND in that area. Once the Syracuse + Pitt were added to the ACC and ND rejected Big Ten's proposal, Delany realized that strategy would no longer work and there was a risk of the ACC boxing out the Big Ten in taking that region so he felt that he needed to react.
Penn State wasn't an immediate flight risk but Penn State joined the Big Ten with the intention of being a major player in the Northeast region. If the Big Ten did nothing and the ACC did establish themselves as the premier conference in the Northeast 20-30 years from now Penn State could have been tempted to reconsider.