Sure. First off, know that the SCOTUS has specifically ruled the Electoral College skew between states is constitutional, but has also ruled that state elections must be one person one vote. This is important because of the mechanics of the Interstate Compact.
The Compact says that member states will allocate their EC vote based on the national popular vote. Non-member states would continue to allocate however they choose (most by winner-take-all statewide popular vote.) So what happens is that member state votes are counted X times (where X = number of member states) while non-member state votes are counted X+1 times. That's an unconstitutional because the state elections for electors (per Bush v Gore) are counted unequally.
Think of a Democratic CA voter in 2000-like scenario (reversed so that the D wins the EC and the R wins the popular vote) having their 55 EC votes flipped to the Republican which also flips the EC result. Their argument will be essentially, "My state should've gone Y but flipped to Z because it unconstitutionally counted non-member states' votes more often."
And that's part of the problem with the Compact, the supporters don't see how badly it could backfire (much like the R's who were knee jerking for all those allocation changes since the 2012 election.) A popular, effective incumbent verses a horrible, elitist challenger eeked out only a 4 point win. When the R's get another plurality, these solidly-D states that have passed the Compact so far will be glad it never got traction.
(Note: also not a lawyer, but I have read extensively on this issue.)