Firstly, your wife is borderline tachy if her resting rate is 90-100 bpm. Tachycardia is defined as a resting rate of 100 or above. Secondly, tachy is a symptom not a disease. There are various causes of tachycardia:
There are two main tachyarrhythmias, ventricular tachy (VT) and supra-ventricular tachys (SVT). Generally these are transient (<1min) and will make you feel pretty unwell if they persist for longer, so it’s unlikely these are the cause.
Autonomic disease can cause tachy something like hyperthyroidism is a common cause (over-active thyroid), these are easily tested for and are usually accompanied by unexplained weight-loss.
Reflex tachycardia can also exist, whereby a sudden drop in BP will be compensated for by an increase in HR. This is caused by a vasodepressor form of syncope (unexplained drop in BP) or by a condition called postural othostatic tachycardia syndrome (POTS) where HR increases when you stand up. Again, these are transient, so are an unlikely cause.
Finally, it may just be physiological. If your wife is very unfit and/or overweight, in order to supply oxygenated blood to her body the heart has to work harder. In this instance, cardio-vascular training and/or weightloss are useful.
It’s important that you rectify the high resting rate as being chronically tachycardic may increase your risk of a heart attack, due to inadequate oxygen supply to the heart via a number of mechanisms that are not really relevant to this forum.
Anyway, hope some of this helps. I am not a doctor (so I would recommend that you see one), but I do work in clinical cardiology.