are these grown from seeds you saved ? I have to try to remember my horticulture education here but theres only few reasons I can give to an early flower bud happening. First of all, it could be genetics . Im sure you can understand the terminology and example. plant Aa and Bb are crossed. you will have any combination of AB,Ab,aB and ab. For now we can just assume the A or B are always tall plants and a and b are always short plants. If you cross its 4 offspring , you can get all tall plants (AB)and all short plants (ab)and the most part of the rest will throw a variety (Ab or aB). Its in the genes that determine plant size, some grow to be short and some tall and a lot of in between. Chances are the genes that went to make those seeds had more short than anything else . A plant also needs x amount of light or lack of to set flower buds. If you started them indoors then set them out ,the amount of light in the indoor location may have been over abundant and then when it went outside to natural light , it was fooled into "thinking" its now in a fall time of year and it decided it was time to set bud. Almost like forcing a bulb , if you take a bulb and want it to grow out of its normal growth season you put it in cold storage for a while then take it out and once it gets the right growth temperature it will grow. Under artificial light and optimal temps it will flower. If one flower got fertilized by the same donor , you may have a strain of fast growing midget tobacco plants. at 8 inches tall, this would definitely not make good leaf or offspring plants, so you may want to destroy that batch.