It is the time when altcoins become unstoppable and massive gains are recoded everyday or in other words the growth in value is simply parabolic. It happens as a part of market cycle and after long bears, it may indicate that the alts were undervalued so a sudden price surge happens to maybe give them real value. Another angle is that because there is huge money flow into btc and eth that money eventually flows into alts causing big pumps called as altseason.