Ultrasonic transducers are almost invariably a tuned LC parallel circuit with the driver pumping it to maintain it, the circulating currents are pretty high, along with the voltages. Voltage depends on the loading, and if the transducer is not too healthy, or if there is no loading then the voltages rise up to really high levels. That the edges are slow is not too much of a concern, even 100pF of capacitor will pass appreciable current at 40kHz if the protection is designed to survive a single cycle of voltage, and dissipate the energy over a 10s period between pulses,
and you slam the same energy in there 400 thousand times, something will melt or simply turn to vapour.
Ultrasonic transducers in industry are generally only a capacitor with high losses, only in echo location do you get the transducer with integrated transformer, as the cables are long, and it is a lot easier to drive a low voltage high current with a transformer to convert impedance at the far end, than to have an ultra low capacitance cable that will be routable easily and survive ultra high voltages as well cheaply. Ultrasonic cleaners to tend to turn transducers into smoke and shrapnel quite easily, I have a few samples of that in cheap cleaners, but the drivers often survive with only blown diode bridges, dead switching transistors and a blown mains fuse. Transducer is just charcoal.