Lesnar walked out post a heated argument with Vince
The Wrestling Observer is reporting that Brock Lesnar's walking out of RAW last week came directly after a heated argument with Vince McMahon himself. It's believed that those near the office could hear the screaming and shouting and were concerned for Vince's well-being. Ultimately Lesnar just stormed off.
It's still not known exactly what the issue was about, but it's assumed to be something to do with his contract or the terms of his renewal. The current deal expires just after Wrestlemania.
Vince reportedly told people backstage during last week's SmackDown tapings that if Lesnar refuses to appear for the remainder of hit dates, that the company will sue for breach of contract. He did however seem to believe that Lesnar would not walk out of Wrestlemania.