Yes eating too many bananas can cause your blood sugar to rise.
The reason that bananas can cause a rise in blood sugar levels is due to the carbs they contain.
So if you're diabetic and need to watch your blood sugar then eating too many bananas can be bad and cause a spike in blood sugar.
So always keep check on your blood sugar levels when eating bananas especially if you're diabetic.
It's usually pretty safe to eat at least 1 or 2 bananas per day when you're a diabetic but I wouldn't eat anymore than 2 bananas if you have problems with high blood sugar levels.