My advice is to have the paratroops in their own separate army instead of as part of a larger one. If this still happens after doing that I would recommend a post with save to the bug report forum.
If your paratroops are part of an army which has been given a garrison order of any kind then I would expect what you are reporting and it is not at all random. All units in a garrison army are always under the control of those objectives and will move on their own to satisfy them.
If your paratroopers are part of an army that has a normal battle plan assigned, you can either do as I suggested by putting them into a separate army or you can select the paratroops and hit control H which removes them from the control of the other battle plan. I, however, find it difficult to control and otherwise less beneficial to try to have more than one plan active within a single army. One could for instance be conducting 4 entirely separate amphibious invasions with a single army, but I find it much easier to have 4 small armies conducting those same operations.
An other possibility is that paratroopers after landing have residual orders which may cause them to begin attacking in some direction without you specifically telling them to do so. This is the case with amphibious invasions. After my marines have landed I usually find it a good idea to cancel their battle plan so that I have more direct control of their actions. After I have brought in further waves I might then set up a new plan for that entire pocket.
The way to tell is that if you look closely at the province in which the paratroops have landed, does it have a border similar to that you see with a frontline battle plan? If so you should be easily able to cancel that plan. I apologize for being so vague but I don't use paratroops so the reply is based on general game mechanics.
