The definition of bullying is -
"the use of force, coercion, hurtful teasing or threat, to abuse, aggressively dominate or intimidate. The behavior is often repeated and habitual. One essential prerequisite is the perception (by the bully or by others) of an imbalance of physical or social power"
This is not the same as being assertive in order to get a job done quickly and effectively!
"Being assertive means being able to stand up for your own or other people's rights in a calm and positive way, without being either aggressive, or passively accepting 'wrong'. Assertive individuals are able to get their point across without upsetting others, or becoming upset themselves".
The Company can employ another person of they wish to - that is nothing to do with you - but they cannot dismiss you unfairly, without suffering consequences (usually financial), so they must feel they have a case if this is really on the cards.
As you are a long time employee, I find it odd that they haven't chatted to you about anything "off the record" rather than going down what sounds like an officious route from the outset. This makes me wonder what sort of relationship you really have with your manager etc.
The trouble is that even if you are right and they are wrong, your trust in them will have diminished so either way I'm afraid you might want to leave - but at least if you play your cards right, you might have some compensation to tide you over until you find a new job.
Good luck!