As in all botlanes, the adc should determine his gameplan and go with it, but in fact, a lot of adcs are opportunistic and do not consider the counterplay. For example, a
vs
lane is literally the one of zoning and positioning. If Cait pokes with Q, zones with traps and DOESN'T trade AAs, she has an advantage as all she needs to do is shoot the net into Draven's face every time he tries to aggress with his MS steroid.
However, if Cait tries to trade, she loses. If she shoots E pointlessly, she loses. If Cait shoves to tower and tries to dance there, she is probably going to get ganked as everyone wants to proc Draven's stacks and Draven can force her E, leaving her open to the jungler. For Cait to truly win against an equally skilled Draven. wave control knowledge is required, which, let's say, is not common. There's also the "thrill" factor - most adc players can't handle the slow and methodical gameplay that is required to beat certain matchups, so whenever they have an advantage they try to force fights which often is not the best thing to do.
So, the lack of the understanding about the matchup, no gameplan and the lack of knowledge usually ruin even advantageous positions. Or, in simple words, your ADC is an idiot.
PS. Absolutely all that I wrote applies to the support as well, aside from wave control which is adc's responsibility.