Bringing together two organisations, two cultures, two ways of doing things, two lots of IT & systems can be a complex business.
Of course the first thing employees will think of is that there may be job cuts. Perhaps there will, perhaps there won't be. Either way, the fear is there and
"Perception is Reality"
Managers have to do the day job as well as the extra work the change brings leaving even less time available to look after their teams. Yet this is absolutely critical for the success of the merger or acquisition.
I once worked for a company that was acquired twice and then merged with a third in the four years I worked there. The changes were not handled well. When I left there were over 80 staff. A few years later not one of those employees was still with the firm.