The simple answer here is: no, there is no reason for a man to take a Viagra tablet thirty minutes before going to get a massage. However, if you are sure that is what is going on, then obviously your husband has his reason(s). (I'm curious about what brought you to count his pills.) It could not logically have anything to do with his bad back. I also question a two hour massage, since one hour is the usual limit. Of course, there are several kinds of massages. Some are strictly for the back, while with others, the massage covers every inch of the body. Some men enjoy "innocently" showing off their erection, so perhaps he likes to show off during his massage? One's imagination can run wild, but Viagra is usually used for one thing. You could ask him about it or even ask to go with him next time, but perhaps that is more than you want to get into.
I don't blame you for being curious. Depending upon the give-and-take you two have with one another, you could either innocently or jokingly ask your husband: Did you ejaculate during your massage today? But then you would want to be ready for the truthful or feigned answer: No, what do you mean by that?! If you don't mind me asking, what sort of sex do you engage in 2-3 times per month? I'm thinking you could work your way into giving your husband a good handjob before he goes to the masseuse.