Attempting to make a stopwatch using two 74hc161n chips with decoders and seven segments
Two problems.

1) Connect your load inputs (A,B,C,D) to ground. Never, ever, leave CMOS inputs floating.

2) Make U4 a 3-input gate, and connect your U5 output to it, as well as the ENP input. What's happening is that your load input acts immediately. It is not gated with the enable inputs. So as soon as you hit 5, U4 goes low and the second counter resets on the next clock edge, rather than waiting 10 counts.