Yes, they sell at Quarterly periods.  They also sold at end of March.

"Executive compensation often includes Restricted Stock Units (RSUs) or performance-linked stock grants that vest right before the fiscal year-end or immediately following the close of a quarter. A large portion is frequently sold automatically on that exact day to cover the immediate income tax liabilities generated by the vesting event."

SEC Rule 10b5-1 contracts, which are pre-scheduled automated trading plans established often years in advance.  This happens to multiple executives per Board of Director rules.

Also they trade outside of blackout windows, which are the weeks leading up to financial disclosure statements.

This happens to tens of thousands of public corporations.  It's actually a good thing, because otherwise insider trading would be [much more] rampant.