Inverter : drive with high impedance. Results on the output
Without knowing what software this is:

Assume that the gates are all ideal. When a1=Z, all three of its attached gates are floating. However, assuming a perfect inverter attached to a1, then one of the FETs in the pass gate will always be conducting. As such, regardless of what voltage a1 floats to, there will be a path for current from Vdd, through R, through the pass gate, to Y. If R has a high resistance, it will be considered a "weak pullup".

In practice, don't do this. Never leave inputs floating, especially FETs.