From the text Shane posted it pretty much seems like he

1) idolized Monty to an almost unhealthy degree (stating he was only there to work with Monty probably isn't the greatest way to convince me that you are focused on the right aspects of driving a series forwards)

2) thinks he can and should fill Monty's role on the project:

mentioning how he was not allowed to keep working the way he and Monty did, when the only reason Monty was allowed to work like that was because he was Monty and he had a reason and an idea as to why he was doing things the way he did (I think Burnie even said something along the lines of Monty being allowed to do things his way because he was Monty, on the podcast at some point [might even have been a while pre-RWBY]) and once Monty was gone it's not surprising for them to change the way the team works to better suit a team working without an artistic master mind such as Monty at the helm.

and 3) Has had a lot of issues letting go of the glory days working with Monty:

To the point where he refuses to acknowledge that the "new" rwby has anything to do with "monty's" rwby despite the people currently in charge of the show having been friends and coworkers of Monty's the same way he was (the exact nature of people's relationships here are obviously not known but Miles and Kerry were at least heavily involved with rwby ever since the beginning of the project as a RT series). [There are a few other parts there too that probably serve as better examples of this but the less we touch on the parts of his private life mentioned in this the better for all parts I think]

All in all the whole thing will blow over in a few days and the one who's going to be stuck with the fallout is Shane, which is a bit saddening as he seems like he could use some help to move on and find something to drive him (from the text his primary motivator before seems to have been working with Monty and learning and replicating his style ["(..)the truth is I came on board to work with Monty. I wanted to be a blessing to him and enhance his workflow or make easier anything and everything he created."]) but in the wake of posting a 36 page attack like this on a former employer to a public forum getting jobs within that field (or as a whole for that matter) is probably going to be a lot more challenging