After capturing Eren and Ymir, Bertolt and Reiner are worried than if they try to escape from the woods they were resting in Titan form, mindless Titans would attack them and kill them. Why werent they worried about this the first time they breached wall Maria and all the mindless titan come inside the walls?
They were always cautious about the surroundings. However, the situation is different in season 2.
As the Colossal Titan, mindless titans are not a problem due to the Colossal's size. However while Berthold and Reiner captured Ymir and Eren, they had to run fast, which means they could not rely on the Colossal Titan. This means that both Berthold and Reiner had to rely on Reiner's Armored Titan to quickly escape the walls. Keep in mind that both of them are trying to run fast because the Survey Corps are after them.
In the first episode,
Reiner only transforms to create a hole in wall Maria, so it is assumed that as soon as the wall was breached, he transformed back into a human.
They were worried, I guess. In Chapter 96 (don't continue to read if you avoid spoiler)
It was showed that Bertholdt's face was kinda shock when he saw Dina Yeager's titan. Reiner seemed worried either. But somehow Dina didn't get any closer to him (or she did? I don't know, the drawing looked ambiguous) and went straight pass to the the wall. Then Reiner quickly rescued Bertholdt and climbed up the wall while some mindless titans desperately trying to reach them from the ground.