After all, if 250 people report to me, probably some of them are going to have opinion A and some are going to have opinion B. If I take a strong public stance in support of A and against B, some of the more nervous B supporters are going to worry I hate them personally and fear I'm a threat to their career - and they're probably going to go to HR about it.
And even if my job doesn't give me any hiring-and-firing powers - if I'm high profile enough that a load of random haters decide they're going to try to get me fired by subjecting my employer to a campaign of harassment, well, now folks like HR and customer services are getting harassed.
Obviously, though, I've never seen a corporation have a blanket policy saying employees can't engage with the political system - that would be pretty bad as a policy. Instead they'll quote policies about 'bringing the company into disrepute' and similar.
The official registration and launch of the campaign was 2 months ago, but he started long before that. If you read his timeline he didn't just wake up one day and decide to run for Congress 2 months ago.