1
general::
Snatch, grasp, as in The beggars kept catching at their coats. [c. 1600] Also see GRASP AT STRAWS.
American Heritage Idioms
2
general::
Phrase(s): catch someone at something [and] catch someone doing something
to discover someone doing something, especially something bad or shameful. • We caught her at her evil deeds. • Don’t let me catch you doing that again!
McGrawhill's American Idioms And Phrasal Verbs